Abstract:

The present invention relates to a method for fluorescent image formation
which can form a highly scratch-resistant fluorescent full-color image
using a colorless fluorescent agent and can freely regulate the tone of
color mixture of a combination of two or more fluorescent colors in order
to impart, to articles, a higher level of forgery preventive function
than a prior art technique and a print having a high level of forgery
preventive function. The invention characterized in that fluorescent inks
are provided that are substantially colorless upon visible light
irradiation and contain organic fluorescent agents which, upon
ultraviolet light irradiation, emit fluorescences in a visible region,
and that two or more fluorescent inks, which emit fluorescences having
mutually different color tones, are deposited on a printing face in its
image formation region according to information on an image to be printed
in a dot matrix manner so that dots of one color do not overlap with dots
of another color.

Claims:

1-6. (canceled)

7. A method for image formation, adapted for the formation of an image
that, upon exposure to ultraviolet light irradiation, emits a plurality
of fluorescent colors and/or a fluorescent color as a mixture of said
fluorescent colors, said method comprising the step of:thermally
transferring two or more organic fluorescent agents, that are
substantially colorless upon visible light irradiation, but on the other
hand, emit fluorescences having mutually different color tones in a
visible region upon ultraviolet irradiation by thermal dye sublimation
transfer according to information on an image to be printed onto a image
formation region in a printing face.

8. The method for image formation according to claim 7, which comprises
the steps of:providing a plurality of thermal transfer sheets comprising
two or more fluorescent dye layers, that emit fluorescences having
mutually different color tones, on a surface of respective substrate
films;putting one of the plurality of thermal transfer sheets provided
respectively with the fluorescent dye layers onto a printing face so that
the fluorescent dye layer faces the image formation region in the
printing face;heating the fluorescent dye layer in the thermal transfer
sheet put on top of the printing face according to information on an
image to be printed to thermally diffuse the organic fluorescent agent
into the image formation region;separating, by the thermal diffusion, the
fluorescent dye layer from the thermal transfer sheet to thermally
transfer the fluorescent dye layer onto the printing face; andthen
subsequently thermally diffusing the organic fluorescent agents
respectively in the other thermal transfer sheets in the same manner as
in the above step onto the same image formation region in which the
fluorescent dye layer has been thermally transferred.

9. The method for image formation according to claim 7, which comprises
the steps of:providing a thermal transfer sheet comprising two or more
fluorescent dye layers, that emit fluorescences having mutually different
color tones, provided subsequently on the surface of an identical
substrate film;putting the thermal transfer sheet provided with the
fluorescent dye layers onto the printing face so that one of the
plurality of fluorescent dye layers provided in the thermal transfer
sheet faces the image formation region in the printing face;heating the
fluorescent dye layer in the thermal transfer sheet put on top of the
printing face according to information on an image to be printed to
thermally diffuse the organic fluorescent agent into the image formation
region;separating, by the thermal diffusion, the fluorescent dye layer
from the thermal transfer sheet to thermally transfer the fluorescent dye
layer onto the printing face; andthen subsequently thermally diffusing
the organic fluorescent agents respectively in the other thermal transfer
sheets in the same manner as in the above step onto the same image
formation region in which the fluorescent dye layer has been thermally
transferred.

10. The method for image formation according to claim 9, which comprises
the steps of:subsequently forming, on the identical substrate film in the
thermal transfer sheet, the fluorescent dye layer and, in addition, a
colorant transfer layer containing a colorant that is visible upon
visible light irradiation to provide a thermal transfer sheet comprising
the fluorescent dye layer and the colorant transfer layer provided on the
identical substrate film surface;putting the provided thermal transfer
sheet onto the printing face in its image formation region so that the
colorant transfer layer faces the image formation region in the printing
face; andheating the colorant transfer layer in the thermal transfer
sheet put on top of the printing face according to information on an
image to be printed to thermally transfer the colorant onto the image
formation region.

11. The method for image formation according to claim 9, which comprises
the steps of:subsequently forming, on the identical substrate film in the
thermal transfer sheet, the fluorescent dye layer and, in addition, a
thermo-fusible black ink layer containing a thermo-fusible black ink to
provide a thermal transfer sheet comprising the fluorescent dye layer and
the thermo-fusible black ink layer provided on the identical substrate
film surface;putting the provided thermal transfer sheet onto the
printing face so that the thermo-fusible black ink layer faces the image
formation region in the printing face; andheating the thermo-fusible
black ink layer in the thermal transfer sheet put onto the printing face
according to information on an image to be printed to thermally transfer
the thermo-fusible black ink onto the image formation region.

12. The method for image formation according to claim 9, which comprises
the steps of:subsequently forming, on the identical substrate film in the
thermal transfer sheet, the fluorescent dye layer and, in addition, a
transferable protective layer to provide a thermal transfer sheet
comprising the fluorescent dye layer and the transferable protective
layer provided on the identical substrate film surface;putting the
provided thermal transfer sheet onto the printing face so that the
transferable protective layer faces the image formation region in the
printing face; andthermally transferring the transferable protective
layer in the thermal transfer sheet put on the printing face onto the
image formation region in the printing face to cover the image formation
region with the protective layer.

13-27. (canceled)

Description:

BACKGROUND OF THE INVENTION

[0001]1. Technical Field

[0002]The present invention relates to a thermal transfer sheet, in which
a fluorescent agent has been incorporated for preventing the
forgery/alteration of important papers, such as securities and paper
money, and cards, such as ID cards and credit cards, and other articles
having great asset values, or for improving the level of design and
amusement, a thermal transfer method using said sheet, and a print
produced using the same. More particularly, the present invention relates
to a thermal transfer sheet that can produce a print which emits
fluorescence with higher intensity and emits a more complicate
fluorescent color and thus is difficult to forge and alter.

[0003]2. Prior Art

[0004]Various methods for preventing the forgery of securities, paper
money, ID cards, credit cards and the like are known. Examples thereof
include a method wherein fine characters or color figure patterns, which
make copying difficult, are printed, a method wherein characters or
images are formed using a transfer foil of gold or silver, which cannot
be reproduced by three primary colors, or special colorants such as inks
having a pastel tone or a pearl tone and fluorescent color inks, and a
method wherein a hologram image, which can be formed only by an advanced
production technique, is provided.

[0005]Further, a method has also been adopted wherein an image, which
cannot be visually perceived under usual service environment, is formed
using a fluorescent agent which does not substantially absorb visible
light and is substantially colorless or white under visible light, but on
the other hand, emits visible fluorescence upon the application of
ultraviolet light, and the print is inspected with an ultraviolet lamp or
the like for the presence of the fluorescent image to judge whether or
not the print is genuine.

[0006]Japanese Patent Laid-Open No. 111800/1987 discloses a thermal
transfer sheet using the above fluorescent agent. Further, Japanese
Patent Laid-Open No. 207452/1996 discloses a thermal transfer sheet
wherein thermally transferable dye layers of three primary colors of red,
blue, green or four colors of the three primary colors and black and, in
addition, a fluorescent color transfer layer containing a thermally
transferable fluorescent dye have been provided in a mutually partitioned
form on a continuous sheet.

[0007]In the prior art techniques, however, even when a fluorescent agent,
which does not substantially absorb visible light and is substantially
colorless or white under visible light, but on the other hand, emits
visible fluorescence upon ultraviolet irradiation, is used, the forgery
of the print is primarily possible by using quite or substantially the
same colorant. In fact, color tones of currently known colorless
fluorescent agents are roughly classified into three colors of red, blue,
and green. For each color, color tones of fluorescent agents are similar
to each other or one another even when they have been produced by
different manufacturers. For example, for colorless fluorescent agents
which emit red light, the emission wavelength is generally around 615 nm.
Therefore, even for an identical color, it is difficult to visually
distinguish one fluorescent agent from another fluorescent agent. For
this reason, when a similar colorless fluorescent agent is available, the
print can be in some cases forged without the use of the colorless
fluorescent agent per se used in the "genuine print."

[0008]Japanese Patent Laid-Open No. 125403/1995 discloses a method for
forming a printed image which emits three or more fluorescent colors upon
exposure to ultraviolet light, wherein images of two or more inks are
printed, by thermal ink transfer using inks containing a fluorescent
pigment or a fluorescent dye as a colorant which emits light upon
exposure to ultraviolet light, on an object so as to partially overlap
with each other.

[0010]In these methods, however, since inks of a plurality of colors are
printed so as to be superimposed on top of each other for the formation
of a fluorescent full-color image, a multilayered structure of ink layers
is formed on a part of the printing face. This poses a problem of
deteriorated scratch resistance of the printed image.

[0011]Further, in the portion where the ink layers of a plurality of
colors have been superimposed on top of each other, the quantity of
ultraviolet light, which reaches the lower ink layer, is smaller than the
quantity of ultraviolet light which reaches the upper ink layer. This
results in lowered emission ability on the lower ink layer side and thus
disadvantageously makes it difficult to regulate the color tone as
desired by mixing of fluorescent colors.

SUMMARY OF THE INVENTION

[0012]Accordingly, an object of the present invention is to provide a
method for fluorescent image formation, which, in order to impart a
higher level of forgery preventive function than the prior art techniques
using colorless fluorescent agents, can form a highly scratch-resistant
fluorescent full-color image and can regulate the color tone, obtained by
color mixing of a combination of two or more fluorescent colors, as
desired, and a print which has a high level of forgery preventive
function.

[0013]The above object can be attained by a method for fluorescent image
formation, adapted for the formation of an image which, upon exposure to
ultraviolet light irradiation, emits a plurality of fluorescent colors
and/or a fluorescent color as a mixture of said plurality of fluorescent
colors, said method comprising the steps of: providing two or more
fluorescent inks respectively containing an organic fluorescent agents
which are substantially colorless upon visible light irradiation, but on
the other hand, emit fluorescences having mutually different color tones
in a visible region upon ultraviolet irradiation; and depositing two or
more fluorescent inks according to information on an image to be printed
in a dot matrix manner onto a printing face in its image formation region
so that dots of one color do not overlap with dots of another color.
According to this method, the above problems can be solved including a
problem that, upon overprinting of fluorescent inks of a plurality of
colors, superimposition of the ink layers of a plurality of colors onto
top of each other leads to deteriorated scratch resistance and a problem
that, upon overprinting of fluorescent inks of a plurality of colors, the
quantity of ultraviolet light, which reaches the lower ink layer, is
smaller than the quantity of ultraviolet light which reaches the upper
ink layer and this results in lowered emission ability on the lower ink
layer side and thus disadvantageously makes it difficult to control the
color tone as desired by mixing of fluorescent colors. Specifically,
color mixing is carried out by the so-called "area gradation" wherein
fluorescent inks of individual colors are deposited in a dot matrix
manner so that dots of one color do not overlap with dots of another
color, and the tone of the fluorescent color is controlled by varying the
area ratio of the color dot groups. Therefore, there is no portion where
the ink layers have been superimposed. Consequently, neither a
deterioration in scratch resistance derived from the superimposition of
the ink layers on top of each other nor a deterioration in emission
ability of the lower ink layer derived from the superimposition of the
ink layers on top of each other takes place. This can facilitate the
control of the tone of the fluorescent color and thus can impart a high
level of forgery preventive property to the print.

[0014]In a preferred embodiment of the present invention as described
above, there is provided a method for image formation that can print two
or more fluorescent colors and can form an image which, upon exposure to
ultraviolet light, emits a plurality of fluorescent colors and/or a
fluorescent color as a mixture of the plurality of fluorescent colors.
This method comprises the steps of: coating two or more thermo-fusible
fluorescent inks, which emit fluorescences having mutually different
color tones, respectively onto the surface of separate substrate film
surfaces, thereby providing a plurality of thermal transfer sheets;
putting one of the plurality of thermal transfer sheets onto a printing
face so that the thermo-fusible fluorescent ink layer faces the printing
face in its image formation region; heating the thermo-fusible
fluorescent ink layer in the thermal transfer sheet put on top of the
printing face according to information on an image to be printed to
thermally transfer the thermo-fusible ink layer onto the image formation
region in a dot matrix manner so that the formed dots do not overlap with
dots of another color which have previously been formed or are to be
formed; separating, by the thermal transfer, the thermo-fusible ink layer
from the thermal transfer sheet to transfer the thermo-fusible ink layer
onto the printing face; and then successively transferring thermo-fusible
fluorescent ink layers respectively in the other thermal transfer sheets
in the same manner as in the above step onto the same image formation
region in which the thermo-fusible ink layer has been thermally
transferred.

[0015]In another preferred embodiment of the above invention, there is
provided a method for image formation that can print two or more
fluorescent colors and can form an image which, upon exposure to
ultraviolet light, emits a plurality of fluorescent colors and/or a
fluorescent color as a mixture of the plurality of fluorescent colors.
This method comprises the steps of: successively coating two or more
thermo-fusible fluorescent inks, which emit fluorescences having mutually
different color tones, onto the surface of identical substrate film to
successively form the plurality of thermo-fusible fluorescent ink layers
onto the identical substrate film surface, thereby providing a thermal
transfer sheet; putting the thermal transfer sheet on top of a printing
face so that one of the thermo-fusible fluorescent ink layer faces the
printing face in its image formation region; heating the thermo-fusible
fluorescent ink layer in the thermal transfer sheet put on top of the
printing face according to information on an image to be printed to
thermally transfer the thermo-fusible fluorescent ink layer onto the
image formation region in a dot matrix manner so that the formed dots do
not overlap with dots of another color which have previously been formed
or are to be formed; separating, by the thermal transfer, the
thermo-fusible ink layer from the thermal transfer sheet to transfer the
thermo-fusible ink layer onto the printing face; and then successively
transferring other thermo-fusible fluorescent ink layers in the thermal
transfer sheet in the same manner as in the above step onto the same
image formation region in which the thermo-fusible ink layer has been
thermally transferred.

[0016]Further, according to a preferred embodiment, in the above thermal
transfer sheet, the thermo-fusible fluorescent ink layer and, in
addition, one or two or more of a colorant transfer layer, a
thermo-fusible black ink layer, and a transferable protective layer are
provided in a face serial manner, and this thermal transfer sheet is used
to thermally transfer a fluorescent image and, in addition, one or two or
more of an image, which can be visually perceived upon exposure to
visible light, a visible image of black ink, and a transferable
protective layer. For example, a thermal sublimation transferable dye
layer or a thermal ink transferable thermo-fusible ink layer may be
provided as the colorant transfer layer. Further, two or more of yellow
(Y), magenta (M), cyan (C) and other color tones may be provided as the
colorant transfer layer in a face serial relationship with other transfer
layers.

[0017]Further, the above object can be attained by the second method for
fluorescent image formation. The second method for image formation
comprises the step of thermally transferring two or more organic
fluorescent agents, which are substantially colorless upon exposure to
visible light, but on the other hand, emit fluorescences different from
each other in color tone upon exposure to ultraviolet light, onto a
printing face in its image formation region by thermal dye sublimation
transfer according to information on an image to be printed. According to
this method, in performing thermal dye sublimation transfer using organic
colorless fluorescent agents, even when overprinting is adopted rather
than the dot matrix method, a high level of forgery preventive property
as attained in the first method can be imparted to prints.

[0018]More specifically, according to the second method of the present
invention, upon the thermal transfer, the matrix in the dye layer stays
on the thermal transfer sheet, and only the colorless fluorescent agent
is sublimated and is diffused into the printing face. Therefore, the
colorless fluorescent agent diffused into the printing face has excellent
invisibility under visible light, and, thus, it is difficult to find the
fact that printing has been performed using a fluorescent agent.

[0019]Further, in the second method according to the present invention,
only the colorless fluorescent agent is thermally diffused into the
printing face, and even in overprinting two or more colors, the
superimposed structure of inks is not formed. Therefore, neither a
deterioration in scratch resistance derived from the superimposition of
the ink layers on top of each other nor a deterioration in emission
ability of the lower ink layer derived from the superimposition of the
ink layers on top of each other takes place.

[0020]Further, the amount of the thermally transferred colorless
fluorescent agent can be regulated on a desired level by varying the
heating energy. The use of a combination of colorless fluorescent agents,
which emit fluorescences different from each other in color tone, can
realize the emission of desired fluorescent colors having various color
tones including white. Further, in this case, the tone of the fluorescent
color produced by color mixing can be infinitely varied. Thus, also in
the second method, as with the first method, a gradational full-color
fluorescent color image can be formed.

[0021]In a preferred embodiment of the second method for image formation
according to the present invention, there is provided a method for image
formation that can print two or more fluorescent colors and can form an
image which, upon exposure to ultraviolet light, emits a plurality of
fluorescent colors and/or a fluorescent color as a mixture of the
plurality of fluorescent colors. This method comprises the steps of:
providing a plurality of thermal transfer sheets, each comprising a
substrate film and, provided on the surface of the substrate film, a
fluorescent dye layer which emits fluorescence having color tone
different from that of fluorescence emitted by a fluorescent dye layer in
other thermal transfer sheet(s); putting one of the plurality of thermal
transfer sheets on top of a printing face so that the fluorescent dye
layer faces the printing face in its image formation region; heating the
fluorescent dye layer in the thermal transfer sheet put on top of the
printing face according to information on an image to be printed to
thermally diffuse the organic fluorescent agent into the image formation
region; separating, by the thermal diffusion, the fluorescent dye layer
from the thermal transfer sheet to thermally transfer the fluorescent dye
layer onto the printing face; and then successively thermally diffusing
organic fluorescent agents in respective other thermal transfer sheets in
the same manner as in the above step onto the same image formation region
where the fluorescent dye layer has been thermally transferred.

[0022]In another preferred embodiment of the second method for image
formation, there is provided a method for image formation that can print
two or more fluorescent colors and can form an image which, upon exposure
to ultraviolet light, emits a plurality of fluorescent colors and/or a
fluorescent color as a mixture of the plurality of fluorescent colors.
This method comprises the steps of: providing a thermal transfer sheet
comprising a substrate film and, provided on substrate film in its
identical surface in a face serial manner, two or more fluorescent dye
layers which emit fluorescences different from each other in color tone;
putting one of the plurality of fluorescent dye layers on top of a
printing face so that the fluorescent dye layer faces the printing face
in its image formation region; heating the fluorescent dye layer in the
thermal transfer sheet put on top of the printing face according to
information on an image to be printed to thermally diffuse the organic
fluorescent agent into the image formation region; separating, by the
thermal diffusion, the fluorescent dye layer from the thermal transfer
sheet to thermally transfer the fluorescent dye layer onto the printing
face; and then successively thermally diffusing organic fluorescent
agents in respective other fluorescent dye layers in the thermal transfer
sheet in the same manner as in the above step onto the same image
formation region where the fluorescent dye layer has been thermally
transferred.

[0023]Further, according to a preferred embodiment, in the above thermal
transfer sheet, the fluorescent dye layer and, in addition, one or two or
more of a colorant transfer layer, a thermo-fusible black ink layer, and
a transferable protective layer are provided in a face serial manner, and
this thermal transfer sheet is used to thermally transfer a fluorescent
image and, in addition, one or two or more of an image, which can be
visually perceived upon exposure to visible light, a visible image of
black ink, and a protective layer. For example, a thermal sublimation
transferable dye layer or a thermal ink transferable thermo-fusible ink
layer may be provided as the colorant transfer layer. Further, two or
more colors selected from yellow (Y), magenta (M), cyan (C) and other
color tones may be properly provided as the colorant transfer layer in a
face serial relationship with other transfer layers.

[0024]Further, the above object can be attained by a first thermal
transfer sheet comprising: a substrate film; and, provided on the surface
of the substrate film, a transfer layer containing a plurality of organic
fluorescent agents, which are substantially colorless upon exposure to
visible light, but on the other hand, emit fluorescences in different
visible regions upon exposure to ultraviolet light. In a preferred
embodiment of the thermal transfer sheet according to the present
invention, regarding the fluorescent color transfer layer, for example, a
thermal ink transfer fluorescent ink layer may be used to thermally
transfer the fluorescent agents together with the ink, or alternatively,
a thermal dye sublimation transfer fluorescent dye layer may be used to
thermally transfer only the fluorescent agents. Further, in another
preferred embodiment, in the above thermal transfer sheet, the
fluorescent color transfer layer and, in addition, one or two or more of
a colorant transfer layer, a thermo-fusible black ink layer, and a
transferable protective layer are provided in a face serial manner, and
this thermal transfer sheet is used to thermally transfer a fluorescent
image and, in addition, one or two or more of an image, which can be
visually perceived upon exposure to visible light, a visible image of
black ink, and a transferable protective layer.

[0025]Further, the above object of the present invention can be attained
by a second thermal transfer sheet comprising: a substrate film; and,
provided on one side of the substrate film in the following order, a
release layer, an intermediate layer, and a heat-sensitive colored layer,
said intermediate layer and said heat-sensitive colored layer containing
fluorescent agents which, upon ultraviolet light irradiation, emit
fluorescence in a visible region. The addition of the fluorescent agent
to both the intermediate layer and the heat-sensitive adhesive layer in
the thermal transfer sheet can enhance the luminance of fluorescence, and
the addition of fluorescent agents different from each other in
fluorescent color respectively to the intermediate layer and the
heat-sensitive adhesive layer can realize the provision of a print which
emits more complicated fluorescent colors and has enhanced level of
forgery/alteration preventive effect and design.

[0026]In a preferred embodiment of the second thermal transfer sheet
according to the present invention, upon ultraviolet light irradiation,
the fluorescent agents emit fluorescences in visible regions having
different color tones. Further, at least one layer selected from the
group consisting of sublimable dye layers of one or more colors selected
from the group consisting of yellow, magenta, cyan, and black colors and
a thermo-fusible black ink layer, and a protective layer may be provided
on the surface of the film in a face serial relationship with the
intermediate layer and the heat-sensitive colored layer. Furthermore, the
intermediate layer and the heat-sensitive colored layer may be formed in
a pattern form.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1A is a typical cross-sectional view of one embodiment of a
thermal transfer sheet usable in the method for image formation according
to the present invention;

[0028]FIG. 1B is a typical cross-sectional view of another embodiment of a
thermal transfer sheet usable in the method for image formation according
to the present invention;

[0029]FIG. 2A is a typical cross-sectional view of one embodiment of a
thermal transfer sheet usable in the method for image formation according
to the present invention;

[0030]FIG. 2B is a typical cross-sectional view of one embodiment of the
construction of a thermal transfer sheet usable in the method for image
formation according to the present invention;

[0031]FIG. 2C is a typical cross-sectional view of another embodiment of
the construction of a thermal transfer sheet usable in the method for
image formation according to the present invention;

[0032]FIG. 2D is a typical cross-sectional view of still another
embodiment of the construction of a thermal transfer sheet usable in the
method for image formation according to the present invention;

[0033]FIG. 2E is a typical cross-sectional view of a further embodiment of
the construction of a thermal transfer sheet usable in the method for
image formation according to the present invention;

[0034]FIG. 3A is a diagram illustrating a basic form of the cross-section
of a thermal transfer sheet according to the present invention;

[0035]FIG. 3B is a diagram showing an embodiment of the formation of an
image on an image-receiving sheet using the thermal transfer sheet
according to the present invention;

[0036]FIG. 3C is a diagram illustrating a basic form of the cross-section
of another embodiment of the thermal transfer sheet according to the
present invention; and

[0037]FIG. 3D is a diagram showing an embodiment of the formation of an
image on an image-receiving sheet using another thermal transfer sheet
according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0038]The present invention will be described in detail.

First Method for Image Formation

[0039]The first method for image formation according to the present
invention can be applied to methods for image formation, wherein a
colorless fluorescent agent is dissolved and dispersed in a certain
matrix to prepare an ink, and the colorless fluorescent agent, together
with the ink, is deposited onto a printing face, for example, thermal ink
transfer and ink jet recording.

[0040]The first method will be described by taking a case, where thermal
ink transfer is used, as a representative example.

[0041]When the first method according to the present invention is carried
out by thermal transfer, a plurality of thermal transfer sheets, wherein
thermo-fusible fluorescent ink layers of two or more colors respectively
formed of thermo-fusible fluorescent inks containing organic fluorescent
agents, which are substantially colorless upon visible light irradiation
and emit fluorescences different from each other in color tone upon
ultraviolet light irradiation, are provided respectively on separate
substrate films, can be used in combination to print two or more
fluorescent colors.

[0042]In this case, an image, which emits a plurality of fluorescent
colors and/or a fluorescent color of a mixture of the plurality of
fluorescent colors upon ultraviolet light irradiation, can be formed by
putting one of the plurality of thermal transfer sheets on top of a
printing face so that the thermo-fusible fluorescent ink layer in the
thermal transfer sheet faces the printing face in its image formation
region, heating the thermo-fusible fluorescent ink layer according to
information on an image to be printed to thermally transfer the
thermo-fusible fluorescent ink onto the printing face in its image
formation region in a dot matrix manner and in such a manner that the
formed dots do not overlap with other color dots which have already been
formed or are to be formed, separating the thermo-fusible fluorescent ink
layer, and then successively thermally transferring the thermo-fusible
fluorescent ink layer in other thermal transfer sheets in the same manner
as described above onto the identical image formation region.

[0043]FIG. 1A is a typical cross-sectional view of an embodiment (101) of
a thermal transfer sheet used in the present invention. The thermal
transfer sheet 101 has a construction such that a thermo-fusible
fluorescent ink layer 2 is provided through a release layer 3 on one side
of a substrate film 1 and a heat-resistant layer 4 for preventing
sticking to a heating element, such as a thermal head, or for improving
slipperiness is provided on the other side of the substrate film 1. The
thermo-fusible fluorescent ink layer is formed by dissolving or
dispersing any colorless fluorescent agent, such as red (R), blue (B), or
green (G), in a thermo-fusible vehicle (matrix) to prepare a solution or
a dispersion and coating the solution or dispersion onto a substrate
film. Upon heating, the colorless fluorescent agent, together with the
thermo-fusible vehicle, can be thermally transferred onto the printing
face. Two or more fluorescent colors can be printed using a plurality of
thermal transfer sheets which have the construction shown in FIG. 1A and
are provided respectively with thermo-fusible fluorescent ink layers each
containing a colorless fluorescent agent which emits a color tone
different from the color tones of fluorescent agents contained in
thermo-fusible fluorescent ink layers in the other thermal transfer
sheets.

[0044]In the present invention, alternatively, two or more fluorescent
colors may be printed using a thermal transfer sheet wherein
thermo-fusible fluorescent ink layers of two or more colors respectively
formed of thermo-fusible fluorescent inks each containing an organic
fluorescent agent, which is substantially colorless upon visible light
irradiation and, upon ultraviolet light irradiation, emits fluorescence
of a color tone different from that of fluorescence emitted from the
fluorescent agent(s) in the other thermo-fusible fluorescent ink
layer(s), are provided in a face serial manner on an identical substrate
film.

[0045]In this case, an image, which emits a plurality of fluorescent
colors and/or a fluorescent color of a mixture of the plurality of
fluorescent colors upon ultraviolet light irradiation, can be formed by
putting one of the thermo-fusible fluorescent ink layers provided in the
thermal transfer sheet on top of the printing face so that the
thermo-fusible fluorescent ink layer faces the printing face in its image
formation region, heating the thermo-fusible fluorescent ink layer
according to information on an image to be printed to thermally transfer
the thermo-fusible fluorescent ink onto the printing face in its image
formation region in a dot matrix manner and in such a manner that the
formed dots do not overlap with other color dots which have already been
formed or are to be formed, separating the thermo-fusible fluorescent ink
layer, and then successively thermally transferring other thermo-fusible
fluorescent ink layers in the identical thermal transfer sheet in the
same manner as described above onto the identical image formation region.

[0046]According to this method wherein two or more fluorescent colors are
printed using one thermal transfer sheet, a construction can be adopted
wherein thermo-fusible fluorescent ink layers of two or more colors are
provided in a face serial manner on a continuous thermal transfer sheet,
the continuous thermal transfer sheet is then reeled in a roll form, the
roll is mounted on a thermal transfer printer, and a plurality of
fluorescent colors are printed from this one reel of the thermal transfer
sheet. This construction is effective in reducing printer size and in
simplifying printer structure. When a colorant transfer layer, such as a
thermo-fusible black ink layer, a sublimable dye layer, or a
thermo-fusible ink layer, a transferable protective layer or the like,
together with the plurality of thermo-fusible fluorescent ink layers, is
provided on the thermal transfer sheet, not only the fluorescent colors
but also conventional colorants, which are visible upon visible light
irradiation, a protective layer and the like can be transferred onto an
identical printing face from the one reel of the thermal transfer sheet.
This is effective in further reducing printer size and simplifying
printer structure. When a color image, which can be visually perceived
upon visible light irradiation, together with the fluorescent color
image, is formed on an identical printing face, the step of transferring
fluorescent agents by thermo-fusible fluorescent ink layers may be
carried out before the step of transferring colorants by colorant
transfer layers such as a thermo-fusible black ink layer, a sublimable
dye layer, and a thermo-fusible ink layer, or vice versa. Preferably,
however, the color image is printed before printing the fluorescent color
image, from the viewpoint of preventing the conventional color image from
hiding the fluorescent color image.

[0047]FIG. 1B is a typical cross-sectional view of one embodiment (102) of
a thermal transfer sheet usable in this case. The thermal transfer sheet
102 has a construction such that thermo-fusible ink layers 7 of yellow
(Y), magenta (M), and cyan (C) which can be visually perceived upon
visible light irradiation (7Y, 7M, 7C), a thermo-fusible black ink layer
5, thermo-fusible fluorescent ink layers 2 of red (R), blue (B), and
green (G) (2R, 2B, 2G), and a transferable protective layer 6 are
provided in a face serial manner on one side of a substrate film 1, that
is, are provided in parallel on an identical substrate film along the
direction of feed of the film at the time of thermal transfer.

[0048]In the thermal transfer sheet 102 shown in FIG. 1B, the
thermo-fusible fluorescent ink layers 2 as well as the thermo-fusible ink
layers 7, the thermo-fusible black ink layer 5, and the transferable
protective layer 6 are provided on the substrate film through a release
layer 3. Further, as with the thermal transfer sheet 101 shown in FIG.
1A, a heat-resistant layer 4 is provided on the backside of the substrate
film in the thermal transfer sheet 102.

[0049]The substrate film constituting the thermal transfer sheet in the
present invention may be formed of a properly selected film material
which has heat resistance and film strength high enough to withstand the
thermal transfer process. The substrate film used in the conventional
thermal transfer sheet may be used in the present invention without any
problem. Specific examples of preferred substrate films include: tissue
papers, such as glassine paper, capacitor paper, and paraffin paper;
stretched or unstretched films or sheets of various plastics, for
example, highly heat-resistant polyesters, such as polyethylene
terephthalate, polyethylene naphthalate, polybutylene terephthalate,
polyphenylene sulfide, polyether ketone, and polyether sulfone,
polypropylene, polycarbonate, cellulose acetate, polyethylene
derivatives, polyvinyl chloride, polyvinylidene chloride, polystyrene,
polyamide, polymethylpentene, and ionomers; and laminate films of a
combination of the above materials.

[0050]The thickness of the substrate film may be properly varied depending
upon materials for the substrate film so that the substrate film has
proper strength, heat resistance or other properties. In general,
however, the thickness is preferably about 1 to 100 μm.

[0051]In the present invention, the thermo-fusible fluorescent ink layer
is a layer which has been formed using one or more fluorescent agents
dissolved or dispersed in a thermo-fusible vehicle and contains at least
an organic fluorescent agent, which is substantially colorless upon
visible light irradiation and, upon ultraviolet light irradiation, emits
fluorescence of visible color, that is, a colorless fluorescent agent,
and a binder resin. In the present invention, the expression
"substantially colorless" means that, upon printing using the fluorescent
agent, even in the case where the ground color of the printing face is
any color tone, the fluorescent agent cannot be visually perceived under
visible light without difficulty and the contents of the print cannot be
distinguished at all.

[0052]Various colorless fluorescent agents are known, and, in the present
invention, any colorless fluorescent agent may be used without particular
limitation, so far as the colorless fluorescent agent is an organic
colorless fluorescent agent, and commercially available organic colorless
fluorescent agents may also be usefully used. Colorless fluorescent
agents are classified into organic colorless fluorescent agents and
inorganic colorless fluorescent agents. In the present invention, organic
colorless fluorescent agents are used. Organic colorless fluorescent
agents can be compatibilized with the binder resin to render the
thermo-fusible ink transparent and thus are highly invisible under
visible light, and, when printing has been carried out using organic
colorless fluorescent agents, during use of the print in a usual manner,
the provision of a fluorescent agent image for preventing the forgery is
less likely to be discovered.

[0053]On the other hand, inorganic colorless fluorescent agents are solid
fine particles and are insoluble in solvents, resins or the like.
Therefore, when a coating is formed using a mixture of the inorganic
colorless fluorescent agent with a binder resin and a solvent, light
scattering among particles occurs and, consequently, the coating is in
many cases seen whitely and is low in the level of colorless,
transparency, and invisibility under visible light. Accordingly, if the
inorganic colorless fluorescent agent is used, the ground color of the
printing face is hided by the inorganic colorless fluorescent agent even
under visible light, and the provision of a fluorescent agent image for
preventing the forgery is likely to be discovered during use of the print
in a usual manner. For the above reason, organic colorless fluorescent
agents are used in the present invention.

[0054]The thermo-fusible fluorescent ink layer is formed of a
thermo-fusible fluorescent ink produced by dissolving or dispersing a
colorless fluorescent agent in a thermally transferable vehicle composed
mainly of a thermo-fusible binder resin, and the colorless fluorescent
agent contained in the thermo-fusible fluorescent ink, together with the
vehicle, is thermally transferred onto the printing face.

[0055]Among commercially available colorless fluorescent inks, those using
organic colorless fluorescent agents may be useful as the thermo-fusible
fluorescent ink. For example, R-50 manufactured by Sinloihi Co., Ltd. may
be mentioned as a red-emitting fluorescent ink, R-70 manufactured by
Sinloihi Co., Ltd. may be mentioned as a green-emitting fluorescent ink,
and MR-30 manufactured by Sinloihi Co., Ltd. may be mentioned as a
blue-emitting fluorescent ink.

[0056]The thermo-fusible fluorescent ink may also be prepared by
dispersing or dissolving a commercially available organic colorless
fluorescent agent in a thermo-fusible binder resin or the like.
Commercially available colorless fluorescent agents include, for example,
red-emitting fluorescent agents such as LC-0001 manufactured by Nippon
Kayaku Co., Ltd., green-emitting fluorescent agents such as EG-502
manufactured by Mitsui Chemicals Inc., and blue-emitting fluorescent
agents such as Uvitex OB manufactured by Ciba-Geigy.

[0057]The thermo-fusible binder resin and other ingredients constituting
the thermally transferable vehicle, together with the organic colorless
fluorescent agent, are transferred onto the printing face and thus
preferably have the highest possible transparency from the viewpoint of
avoiding a reduction in visibility of the image in the printed face. In
particular, the thermo-fusible binder resin as the main component of the
vehicle is preferably substantially colorless and transparent under
visible light.

[0059]If necessary, other ingredients may be incorporated into the
thermo-fusible fluorescent ink layer. For example, the incorporation of
inorganic fine particles of silica or the like into the thermo-fusible
fluorescent ink layer can improve the transferability of the ink layer.

[0060]The content ratio of the colorless fluorescent agent to the binder
resin in the thermo-fusible fluorescent ink layer may be properly
determined according to required properties. The intensity of the
fluorescent color emitted upon ultraviolet light irradiation depends upon
the content of the colorless fluorescent agent. Therefore, the higher the
content of the colorless fluorescent agent, the higher the vividness of
the rendered color. Since, however, colorless fluorescent agents are more
expensive than conventional colorants, the use of these colorless
fluorescent agents in an unnecessarily large amount is uneconomical.
Further, when the colorless fluorescent agent does not have high
compatibility with the binder resin, excessively increasing the content
of the colorless fluorescent agent poses a problem such as precipitation
of the colorless fluorescent agent in the thermo-fusible fluorescent ink
layer. For the above reason, the content of the colorless fluorescent
agent in the thermo-fusible fluorescent ink is preferably about 0.01 to
50% by weight, particularly preferably about 0.1 to 20% by weight, based
on the whole thermo-fusible fluorescent ink, and the content of the
binder resin is preferably about 50 to 99.99% by weight, particularly
preferably about 80 to 99.9% by weight, based on the whole thermo-fusible
fluorescent ink.

[0061]The thickness of the thermo-fusible fluorescent ink layer is
generally 0.2 to 5 μm, preferably 0.4 to 3 μm. When the thickness
of the thermo-fusible fluorescent ink layer is less than 0.2 μm, the
level of the evenness of the layer thickness is lowered leading to uneven
color development. On the other hand, when the thickness of the
thermo-fusible fluorescent ink layer exceeds 5 μm, the layer
transferability is deteriorated, leading to a fear of the thermo-fusible
fluorescent ink layer being transferred also onto a region other than the
desired region.

[0062]The thermo-fusible fluorescent ink layer may be formed on the
substrate film by dissolving or dispersing the colorless fluorescent
agent, the binder resin and optionally other ingredients in a single
solvent or a mixed solvent composed of two or more solvents selected from
toluene, methyl ethyl ketone, ethyl acetate, isopropanol and the like to
prepare a coating liquid, coating the coating liquid onto the substrate
film by a conventional method, such as gravure coating, gravure reverse
coating, or roll coating, and drying the coating.

[0063]Alternatively, the thermo-fusible fluorescent ink layer may be
formed by heat melting a coating material comprising the colorless
fluorescent agent, the thermo-fusible binder resin, and optionally other
ingredients, instead of the dissolution of the coating material in the
solvent, and coating the melt onto the substrate film by a conventional
method, such as thermo-fusible coating, hot lacquer coating, gravure
coating, gravure reverse coating, or roll coating, and cooling the
coating.

[0064]In the thermal transfer sheet used in the first method according to
the present invention, in addition to the thermo-fusible fluorescent ink
layer, colorant layers of yellow, magenta, cyan, black and the like may
be provided in a face serial manner. Sublimable dye-containing dye layers
and thermo-fusible ink layers may be used as the colorant layer.

[0065]The dye layer is formed of a sublimable dye dissolved or dispersed
in a non-transferable vehicle composed mainly of a non-thermo-fusible
binder resin, and only the sublimable dye contained in the dye layer can
be thermally transferred onto the printing face. Since the sublimable dye
is highly transparent, even in the case where a fluorescent color image
is first formed in the printing face in its image formation region
followed by the formation of a visible image using a sublimable dye in
the identical image formation region, advantageously, the fluorescent
color image is not hided by the visible image.

[0067]The non-thermo-fusible binder resin and other ingredients in the dye
layer may be those used in the non-transferable vehicle in a
sublimatlon-type fluorescent dye layer which will be described later.

[0068]The content of the sublimable dye in the dye layer is generally
about 5 to 90% by weight, preferably about 10 to 70% by weight, based on
the whole dye layer. The thickness of the dye layer is generally 0.2 to 5
μm, preferably 0.4 to 2 μm.

[0069]The dye layer may be formed on the substrate film by dissolving or
dispersing the sublimable dye, the binder resin and optionally other
ingredients in a single solvent or a mixed solvent composed of two or
more solvents selected from toluene, methyl ethyl ketone, ethyl acetate,
isopropanol and the like to prepare a coating liquid, coating the coating
liquid onto the substrate film by a conventional method, such as gravure
coating, gravure reverse coating, or roll coating, and drying the
coating.

[0070]The thermo-fusible ink layer is formed of a thermo-fusible color ink
comprising a colorant, such as yellow, magenta, cyan, or black, and a
thermo-fusible vehicle. The thermo-fusible vehicle is composed mainly of
a thermo-fusible binder and optionally contains other ingredients.
Colorants usable herein include organic or inorganic pigments and dyes.

[0072]In particular, when a thermo-fusible black ink layer is formed,
carbon black is preferably used as the black colorant. Among organic or
inorganic pigments and dyes, carbon black has good properties as a
recording material, such as satisfactory color density and neither
discoloration nor fading upon exposure to light, heat, high temperature
and the like, and thus can print high-density and clear characters and
symbols.

[0073]Any of the following binders resins 1) to 5) is preferably used as
the thermo-fusible binder from the viewpoint of the adhesion to the
image-receiving sheet and the scratch resistance:

[0074]1) acrylic resin;

[0075]2) acrylic resin+chlorinated rubber;

[0076]3) acrylic resin+vinyl chloride/vinyl acetate copolymer resin;

[0077]4) acrylic resin+cellulosic resin; and

[0078]5) vinyl chloride/vinyl acetate copolymer resin.

[0079]Instead of the binder resin, wax or the like may be used. Further,
wax and the like may be added to the above binder resin. Representative
examples of waxes include microcrystalline wax, carnauba wax, and
paraffin wax. Further, Fischer-Tropsh wax, various low-molecular weight
polyethylene waxes, Japan wax, beeswax, spermaceti, insect wax, wool wax,
shellac wax, candelilla wax, petrolatum, partially modified wax, fatty
esters, fatty amides, and other various waxes may also be used.

[0080]The thermo-fusible ink layer may be formed on the substrate film by
the same method as used in the formation of the thermo-fusible
fluorescent ink layer, that is, by dissolving or dispersing necessary
materials in a solvent to prepare a coating liquid, coating the coating
liquid onto a substrate film, and drying the coating, or by heat melting
necessary materials, coating the melt onto a substrate film, and cooling
the coating. The thickness of the thermo-fusible ink layer is determined
based on the relationship between necessary color density and heat
sensitivity and is generally preferably in the range of about 0.2 to 10
μm.

[0081]In the thermal transfer sheet used in the present invention, in
addition to the thermo-fusible fluorescent ink layer, a transferable
protective layer may be provided in a face serial manner. After the
completion of the formation of an image on the printing face, the
transferable protective layer is transferred onto the image formation
region. The protective layer may be formed of various resins which have
hitherto been used as a protective layer for thermally transferred
images. Examples of resins include polyester resins, polystyrene resins,
acrylic resins, polyurethane resins, acrylated urethane resins, the above
resins modified with silicone, mixtures of the above resins, ionizing
radiation-curable resins, and ultraviolet screening resins.

[0082]The protective layer containing an ionizing radiation-curable resin
is excellent particularly in plasticizer resistance and scratch
resistance. Conventional ionizing radiation-curable resins may be used,
and an example thereof is a composition which comprises a radically
polymerizable polymer or oligomer and optionally a photopolymerization
initiator and is crosslink-polymerizable by the application of an
ionizing radiation such as electron beams or ultraviolet light.

[0083]In general, the thickness of the protective layer is preferably in
the range of about 0.5 to 10 μm although the thickness varies
depending upon the resin for the protective layer.

[0084]The protective layer may have a multilayer structure comprising a
plurality of layers different from each other in function, and, for
example, an adhesive layer may be provided on the outermost surface of
the protective layer. The adhesive layer may be formed of a resin having
good adhesion upon heating, for example, acrylic resin, vinyl chloride
resin, vinyl acetate resin, vinyl chloride/vinyl acetate copolymer resin,
polyester resin, or polyamide resin. The thickness of the adhesive layer
is generally in the range of about 0.1 to 5 μm.

[0085]The transferable protective layer may be formed by dissolving or
dispersing a resin for a protective layer in a single solvent or a mixed
solvent composed of two or more solvents selected from toluene, methyl
ethyl ketone, ethyl acetate, isopropanol and the like to prepare a
coating liquid for a protective layer, coating the coating liquid onto a
substrate film or a stripping layer by a conventional method, such as
gravure coating, gravure reverse coating, or roll coating, and drying the
coating. When the ionizing radiation-curable resin is used, after drying
the coating, an ionizing radiation such as ultraviolet light or electron
beams is applied to cure the coating.

[0086]When the transferable protective layer has a multilayer structure
having an additional layer such as the adhesive layer, a method may be
used wherein a coating liquid, for a protective layer, containing a resin
for a protective layer, a thermally adhesive resin-containing coating
liquid for an adhesive layer, and a coating liquid(s) for an optional
additional layer(s) are previously prepared and are coated in a
predetermined order onto a substrate film or a release layer followed by
drying. A proper primer layer may be formed between the layers.

[0087]In order to easily transfer the thermo-fusible fluorescent ink
layer, the thermo-fusible black ink layer, or the transferable protective
layer from the thermal transfer sheet to the image-receiving sheet, the
release layer is provided between the substrate film and these layers.
The thermo-fusible fluorescent ink layer, the thermo-fusible black ink
layer, or the transferable protective layer is separated at the interface
of these layers and the release layer and is transferred onto the
image-receiving sheet, and the release layer stays on the substrate film.
This release layer is particularly effective when the substrate film has
been subjected to easy-adhesion treatment (adhesion improvement
treatment) such as corona discharge treatment.

[0088]The release layer may be formed of, for example, urethane resin,
polyvinyl acetal resin, or a mixture of these resins. The release layer
may be formed in the same manner as used in the formation of the
thermo-fusible fluorescent ink layer or the transferable protective
layer, that is, by dissolving or dispersing a resin for a release layer
in a solvent to prepare a coating liquid and coating the coating liquid
onto a substrate film by a conventional method. In general, the thickness
of the release layer is preferably about 0.1 to 5 μM.

[0089]A heat-resistant layer is preferably provided on the backside of the
substrate film, that is, on the substrate in its side remote from the
thermo-fusible fluorescent ink layer, from the viewpoints of preventing
fusing of the sheet to a heating element, such as a thermal head,
improving sheet feeding, and preventing blocking of the backside to the
frontside of the thermal transfer sheet according to the present
invention upon winding of the sheet in a roll form.

[0090]The heat-resistant layer may be formed of, for example, a resin such
as a curable silicone oil, a silicone resin, a fluororesin, an acrylic
resin, or a polyvinylbutyral resin, or a cured product thereof. In some
cases, surfactants or various fillers are added to the above material,
for example, for regulating the slip property of the heat-resistant
layer. The heat-resistant layer may be formed in the same manner as used
in the formation of the thermo-fusible fluorescent ink layer or the
transferable protective layer, that is, by dissolving or dispersing a
material for a heat-resistant layer in a solvent to prepare a coating
solution and coating the coating liquid onto a substrate film by a
conventional method.

[0091]Next, a method for forming a fluorescent color image (a forgery
preventive mark) using the thermal transfer sheet will be described. When
the first method is carried out by thermal ink transfer, an image, which
emits a plurality of fluorescent colors and/or a fluorescent color of a
mixture of the plurality of fluorescent colors upon ultraviolet light
irradiation, can be formed by putting one of a plurality of thermal
transfer sheets, each provided with a single or two or more of
thermo-fusible fluorescent ink layers, on top of a printing face so that
the thermo-fusible fluorescent ink layer in the thermal transfer sheet
faces the printing face in its image formation region, heating the
thermo-fusible fluorescent ink layer according to information on an image
to be printed to thermally transfer the thermo-fusible fluorescent ink
onto the printing face in its image formation region in a dot matrix
manner and in such a manner that the formed dots do not overlap with
other color dots which have already been formed or are to be formed,
separating the thermo-fusible fluorescent ink layer, and then
successively thermally transferring the thermo-fusible fluorescent ink
layer in the identical or other thermal transfer sheet in the same manner
as described above onto the identical image formation region.

[0092]The first method is the so-called "area gradation," and, as shown in
FIG. 1B, fluorescent inks of two or more colors are thermally transferred
onto the printing face in a dot form while regulating the transferred
area for each color tone and in such a manner that dots of one color do
not overlap with dots of other colors. According to this method, the
fluorescent color of each transferred dot is microscopically a single
color. However, when the transferred area unit of each dot is
satisfactorily reduced, the color is perceived by the eye of the human as
a fluorescent color produced by additive color mixing of colors according
to the area ratio of the color dot groups. When this method is applied to
thermal ink transfer, the use of a thermal head having a resolution equal
to or higher than about 150 DPI loaded in conventional thermal printers
suffices for the visual perception of the additively mixed fluorescent
color. The area of each dot group can be regulated by increasing or
reducing any one of or both the number of dots and the area per dot.

[0093]Color tones of colorless fluorescent agents are roughly classified
into three colors, red, blue, and green. According to the method of the
present invention, rather than any one of these color tones, two, three
or more color tones are used to form an image that emits a plurality of
fluorescent colors which are indistinguishable under visible light,
making it difficult to perform forgery.

[0094]According to the present invention, a higher level of forgery
preventive property can be imparted. Specifically, a fluorescent color
having any desired color tone including white light can be produced by
mixing red, blue, and green together after properly regulating the
intensity of each color. Ordinary colorants absorb visible light and emit
complementary color. On the other hand, colorless fluorescent agents
absorb ultraviolet light and emit fluorescence of visible color, and the
color mixture follows the law of additive color mixture. Therefore, the
use of a combination of colorless fluorescent agents, which emit
fluorescences different from each other or one another in color tone, can
freely produce fluorescent colors having a variety of color tones
including white. The color tone of fluorescent colors produced by color,
mixing can be infinitely varied. This can realize the formation of
gradational full-color fluorescent color images. In the present
invention, by virtue of these properties, a plurality of colorless
fluorescent agents may be used to form an image which emits a plurality
of fluorescent colors including a fluorescent color as a color mixture,
and, thus, a high level of forgery preventive property can be imparted.
Further, since fully copying the color tone of a fluorescent color as a
certain color mixture is difficult without learning the types and
blending ratio of colorless fluorescent agents used. Therefore, as
compared with the use of only a colorless fluorescent agent of a single
color, the level of difficulty of forgery can be significantly enhanced.
In particular, the formation of a gradational full-color fluorescent
color image using a combination of three primary colors, red, blue, and
green is preferred because a very high level of forgery preventive
property can be imparted.

[0095]Further, since the fluorescent color image produced according to the
present invention is formed using a combination of a plurality of
fluorescent agents, a complicate fluorescence absorption spectrum can be
produced. Furthermore, when a conventional image, which can be visually
perceived under visible light, is printed so as to be superimposed on the
fluorescent color image formation region, a complicate
ultraviolet-visible absorption spectrum or fluorescent-visible absorption
spectrum can be produced in the image formation region. Accordingly, as
useful forgery preventive means, a method may be adopted wherein thermal
transfer is carried out using a predetermined combination of colorless
fluorescent agents and optionally a predetermined colorant(s), the form
of an ultraviolet-visible absorption spectrum and/or the form of a
fluorescent-visible absorption spectrum are utilized as "key" information
for the prevention of forgery, and the form of the ultraviolet-visible
absorption spectrum and/or the form of the fluorescent-visible absorption
spectrum are detected to judge whether or not the print is genuine.

Second Method for Image Formation

[0096]Thermal dye sublimation transfer sheets usable in the second method
for image formation according to the present invention may be the same as
the thermal transfer sheets shown in FIGS. 1A and 1B usable in the first
method according to the present invention, except that the thermo-fusible
fluorescent ink layer in the thermal transfer sheets shown in FIGS. 1A
and 1B has been replaced with a fluorescent dye layer containing a highly
sublimable colorless fluorescent agent. In this case, however, regarding
the fluorescent dye layer, there is no need to thermally transfer the
whole vehicle-containing dye layer. Therefore, the provision of the dye
layer through a release layer on a substrate film is not required.
Instead, preferably, the substrate film is subjected to adhesion
improvement treatment such as corona discharge treatment of the substrate
film or the interposition of a primer layer between the dye layer and the
substrate, from the viewpoint of improving the adhesion between the
non-transferable vehicle and the substrate film.

[0097]The fluorescent dye layer is formed by dissolving or dispersing a
sublimable colorless fluorescent agent in a non-transferable vehicle and
coating the solution or dispersion onto a substrate film. Upon heating,
only the organic fluorescent agent can be thermally diffused from the
fluorescent color transfer layer into the printing face, and the
non-transferable vehicle stays on the thermal transfer sheet.

[0098]The highly sublimable organic colorless fluorescent agent may be
those exemplified in the first method. The non-transferable vehicle is
composed mainly of a non-thermo-fusible binder resin and optionally
contains other ingredients.

[0099]The non-thermo-fusible binder resin is not fused at the heating
temperature in the thermal transfer process. Specific examples of
non-thermo-fusible binder resins include those commonly used as binder
resins for a sublimable dye layer, for example, cellulosic resins such as
ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose,
methylcellulose, cellulose acetate, and cellulose acetate butyrate; vinyl
resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral,
polyvinyl acetal, and polyvinylpyrrolidone, poly(meth)acrylamides;
polyurethane resins; polyamide resins; polyester resins; and mixtures of
these resins. Among them, cellulosic, vinylacetal, vinylbutyral, and
polyester resins are preferred from the viewpoints of transferability of
dyes and the like.

[0100]The sublimable fluorescent dye layer may optionally contain other
ingredients. For example, a release agent, such as a silicone oil or
polyethylene wax, may be incorporated into the sublimable fluorescent dye
layer from the viewpoint of regulating friction between the fluorescent
dye layer and the object or preventing blocking in a wound state.

[0101]As with the thermo-fusible fluorescent ink used in the first method,
the content ratio of the colorless fluorescent agent to the binder resin
in the fluorescent dye layer used in the second method may be properly
determined according to properties required. The content of the colorless
fluorescent agent is preferably about 0.1 to 80% by weight, particularly
preferably about 1 to 50% by weight, based on the whole fluorescent dye
layer. The content of the binder resin is preferably about 20 to 99.9% by
weight, particularly preferably about 50 to 99% by weight, based on the
whole fluorescent dye layer. The content of the colorless fluorescent
agent in the fluorescent dye layer is preferably larger than the content
of the colorless fluorescent agent in the thermo-fusible fluorescent ink
layer. The reason for this is as follows. In the case of thermal dye
sublimation transfer, the fluorescent agent in the transfer layer is not
completely transferred onto the object, and a part of the fluorescent
agent stays in the transfer layer. Therefore, in order to form a sharp
fluorescent color image, the content of the fluorescent agent in the
transfer layer should be large.

[0102]As with the thickness of the thermo-fusible fluorescent ink layer,
the thickness of the fluorescent dye layer is generally 0.2 to 5 μm,
preferably 0.4 to 3 μm.

[0103]The fluorescent dye layer may be formed on the substrate film by
dissolving or dispersing the colorless fluorescent agent, the binder
resin and optionally other ingredients in a single solvent or a mixed
solvent composed of two or more solvents selected from toluene, methyl
ethyl ketone, ethyl acetate, isopropanol and the like to prepare a
coating liquid, coating the coating liquid onto the substrate film by a
conventional method, such as gravure coating, gravure reverse coating, or
roll coating, and drying the coating.

[0104]In the formation of a fluorescent image according to the second
method, as described above, even when two or more colors are printed on
the printing face in its identical portion so as to be superimposed on
top of each other, a fluorescent full-color image having excellent
scratch resistance can be produced, and the color tone of the image can
be easily controlled. Further, in the second method, fluorescent printing
may be carried out in a dot matrix manner as used in the first method.
Also in this case, a fluorescent full-color image having excellent
scratch resistance can be formed, and the color tone of the image can be
easily controlled.

[0105]In the second method, particularly when a fluorescent image is
formed on a printing face by thermal dye sublimation transfer using a
thermal dye sublimation fluorescent dye layer, the formation of a pattern
is possible. Therefore, an image can be formed which emits fluorescence
as a color mixture of two or more fluorescent agents and has a
gradationally smoothly changed color density. This can realize the
formation of a fluorescent image which is highly difficult to forge.

[0106]Next, the thermal transfer sheet used in the present invention will
be described.

First Thermal Transfer Sheet

[0107]FIG. 2A is a typical cross-sectional view of one embodiment (101) of
the thermal transfer sheet usable in the thermal transfer method
according to the present invention. The construction of the thermal
transfer sheet 101 is such that a thermo-fusible transfer fluorescent ink
layer 2a is provided on one side of a substrate film 1 through a release
layer 3 and a heat-resistant layer 4 is provided on the substrate film 1
in its side remote from the thermo-fusible transfer fluorescent ink layer
2a, from the viewpoints of preventing sticking to a heating element, such
as a thermal head, and improving slipperiness. The thermal ink transfer
fluorescent ink layer is formed by properly selecting two or more
fluorescent agents from red (R), blue (B), green (G) and the like,
dissolving or dispersing the two or more selected fluorescent agents in a
thermo-fusible vehicle, and coating the solution or dispersion onto a
substrate film. Upon heating, a mixture of a plurality of fluorescent
agents, together with the ink, can be thermally transferred onto the
printing face to print a fluorescent color as a color mixture.

[0108]Further, as described above, in the present invention, a method may
also be adopted wherein a fluorescent color as a mixture of two or more
colors is printed using a thermal transfer sheet wherein a fluorescent
color transfer layer and, in addition, colorant transfer layers, such as
a thermo-fusible ink layer, a thermo-fusible black ink layer, and a
sublimable dye layer, and/or a transferable protective layer are provided
in a face serial manner on a substrate film in an identical thermal
transfer sheet. According to this method, not only a fluorescent color
but also a conventional colorant, which can be visually perceived upon
visible light irradiation, and/or a protective layer and the like can be
transferred onto an identical printing face from one reel of a thermal
transfer sheet, by providing a fluorescent color transfer layer and, in
addition, a colorant transfer layer of a single color or colorant
transfer layers of two or more colors and/or a transferable protective
layer in a face serial manner on a substrate film in a continuous thermal
transfer sheet, then reeling the continuous thermal transfer sheet in a
roll form, and mounting the roll on a thermal transfer printer. This
construction is effective in reducing printer size and in simplifying
printer structure. When a fluorescent color image, which can be visually
perceived only under ultraviolet light, together with a color image,
which can be visually perceived under visible light, is formed on an
identical printing face, the step of transferring a fluorescent agent
using the fluorescent color transfer layer may be carried out before or
after the step of transferring a colorant using the colorant transfer
layer such as the thermo-fusible ink layer, the thermo-fusible black ink
layer, or the sublimable dye layer. Preferably, however, the color image
is printed before printing the fluorescent color image, from the
viewpoint of preventing the conventional color image from hiding the
fluorescent color image.

[0109]FIGS. 2B to 2E are typical cross-sectional views of embodiments (102
to 105) of the construction of the thermal transfer sheet usable in this
case. The thermal transfer sheet 102 shown in FIG. 2B has a construction
such that three dye layers, i.e., a dye layer containing a sublimable dye
of yellow (Y), a dye layer containing a sublimable dye of magenta (M),
and a dye layer containing a sublimable dye of cyan (C) (5Y, 5M, 5C), and
a thermal ink transfer fluorescent ink layer 2a are provided in a face
serial manner on one side of a substrate film 1, that is, on an identical
substrate film, in parallel along the direction of feed of the film at
the time of the thermal transfer. In the thermal transfer sheet 102 shown
in FIG. 2B, the dye layers (5Y, 5M, 5C) are provided directly on the
substrate film 1. On the other hand, the thermal ink transfer fluorescent
ink layer 2a adjacent to the dye layers is provided on the substrate film
through a release layer 3. As with the thermal transfer sheet 101 shown
in FIG. 2A, in the thermal transfer sheet 102, a heat-resistant layer 4
is provided on the backside of the substrate film.

[0110]A thermal transfer sheet 103 shown in FIG. 2C has a construction
such that a release layer 3 is provided on one side of a substrate film 1
and, in addition, three thermo-fusible ink layers, i.e., a thermo-fusible
ink layer containing a colorant of yellow (Y), a thermo-fusible ink layer
containing a colorant of magenta (M), and a thermo-fusible ink layer
containing a colorant of cyan (C) (6Y, 6M, 6C), a thermo-fusible black
ink layer 7, and a thermal ink transfer fluorescent ink layer 2a are
provided in a face serial manner on the release layer 3. Further, as with
the thermal transfer sheet 101 shown in FIG. 2A, in the thermal transfer
sheet 103, a heat-resistant layer 4 is provided on the backside of the
substrate film.

[0111]A thermal transfer sheet 104 shown in FIG. 2D has a construction
such that three dye layers, i.e., a dye layer containing a sublimable dye
of yellow (Y), a dye layer containing a sublimable dye of magenta (M),
and a dye layer containing a sublimable dye of cyan (C) (5Y, 5M, 5C), a
thermal ink transfer fluorescent ink layer 2a, and a transferable
protective layer 8 are provided in a face serial manner on one side of a
substrate film 1. In the thermal transfer sheet 104 shown in FIG. 2D, the
dye layers (5Y, 5M, 5C) are provided directly on the substrate film 1. On
the other hand, the thermal ink transfer fluorescent ink layer 2a and the
transferable protective layer 8 adjacent to the dye layers are provided
on the substrate film through a release layer 3. As with the thermal
transfer sheet 101 shown in FIG. 2A, in the thermal transfer sheet 104, a
heat-resistant layer 4 is provided on the backside of the substrate film.

[0112]A thermal transfer sheet 105 shown in FIG. 2E has a construction
such that three dye layers, i.e., a dye layer containing a sublimable dye
of yellow (Y), a dye layer containing a sublimable dye of magenta (M),
and a dye layer containing a sublimable dye of cyan (C) (5Y, 5M, 5C), a
thermo-fusible black ink layer 7, a thermal dye sublimation transfer
fluorescent dye layer 2b containing two or more colorless fluorescent
agents, and a transferable protective layer 8 are provided in a face
serial manner on one side of a substrate film 1.

[0113]In the thermal transfer sheet 105 shown in FIG. 2E, the dye layers
(5Y, 5M, 5C) and the thermal dye sublimation transfer fluorescent dye
layer 2b are provided directly on the substrate film 1. On the other
hand, the thermo-fusible black ink layer 7 and the transferable
protective layer 8 adjacent to the dye layers are provided on the
substrate film through a release layer 3. Further, as with the thermal
transfer sheet 101 shown in FIG. 2A, in the thermal transfer sheet 105, a
heat-resistant layer 4 is provided on the backside of the substrate film.

[0114]The fluorescent color transfer layer in the thermal transfer sheet
105 is the thermal dye sublimation transfer layer which has been formed
by dissolving or dispersing a highly sublimable and thermally sublimation
transferable organic colorless fluorescent agent in a non-transferable
vehicle and coating the solution or dispersion onto a substrate film.
Upon heating, only the fluorescent agent can be thermally transferred
from the fluorescent color transfer layer to a printing face, and, the
non-transferable vehicle stays on the thermal transfer sheet.

[0115]The dye layer is formed by dissolving or dispersing a sublimable
dye, which has a color under visible light, in a non-transferable vehicle
and coating the solution or dispersion onto a substrate film. Upon
heating, only the dye can be thermally transferred from the dye layer
onto a printing face, and the non-transferable vehicle stays on the
thermal transfer sheet.

[0116]In the sublimation fluorescent color transfer layer and dye layer,
there is no need to thermally transfer the whole vehicle-containing
transfer layer. This can eliminate the need to provide the sublimation
fluorescent color transfer layer and dye layer on the substrate film
through a release layer. Instead, preferably, the substrate film is
subjected to adhesion improvement treatment such as corona discharge
treatment of the substrate film or the interposition of a primer layer
between the fluorescent layer and dye layer and the substrate, from the
viewpoint of improving the adhesion between the non-transferable vehicle
and the substrate.

[0117]Next, each element constituting the first thermal transfer sheet
according to the present invention will be described in detail.

[0118]The substrate film, the colorless fluorescent agent, the
thermo-fusible fluorescent ink, the thermal ink transfer fluorescent ink
layer, the dye layer, the sublimable dye layer, the thermo-fusible ink
layer, the transferable protective layer, and the release layer
constituting the first thermal transfer sheet according to the present
invention may be the same as those in the thermal transfer sheet used in
the above method for image formation and may be formed in the same manner
as used in the formation of the thermal transfer sheet used in the above
method for image formation.

[0119]The fluorescent agent transfer layer constituting the thermal
transfer sheet according to the present invention is formed by dissolving
or dispersing fluorescent agents in a vehicle and coating the solution or
dispersion. The fluorescent agent transfer layer contains at least two or
more fluorescent agents, which are substantially colorless upon visible
light irradiation, but on the other hand, upon ultraviolet light
irradiation, emit fluorescence of a visible color, that is, colorless
fluorescent agents, and a binder resin.

[0120]The thermal ink transfer fluorescent ink layer in the fluorescent
agent transfer layer is formed of a thermo-fusible fluorescent ink
comprising an organic colorless fluorescent agent dissolved or dispersed
in a thermally transferable vehicle composed mainly of a thermo-fusible
binder resin, and the fluorescent agent in the fluorescent color transfer
layer, together with the vehicle, can be thermally transferred onto a
printing face.

[0121]The thermal dye sublimation transfer fluorescent dye layer is formed
by dissolving or dispersing a sublimable colorless fluorescent agent in a
non-transferable vehicle and coating the solution or dispersion onto a
substrate film. Upon heating, only the organic fluorescent agent can be
thermally diffused from the fluorescent color transfer layer to a
printing face, and the non-transferable vehicle stays on the thermal
transfer sheet.

[0122]Organic colorless fluorescent agents, which are highly sublimable
and are usable in the thermal dye sublimation transfer, include
fluorescent agents exemplified above as usable in the thermal ink
transfer fluorescent ink layer. The non-transferable vehicle is composed
mainly of a non-thermo-fusible binder resin and optionally contains other
ingredients.

[0123]The non-thermo-fusible binder resin is not fused at the heating
temperature in the thermal transfer process. Specific examples of
non-thermo-fusible binder resins include those commonly used as binder
resins for a sublimable dye layer, for example, cellulosic resins such as
ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose,
methylcellulose, cellulose acetate, and cellulose acetate butyrate; vinyl
resins such as polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral,
polyvinyl acetal, and polyvinylpyrrolidone, poly(meth)acrylamides;
polyurethane resins; polyamide resins; polyester resins; and mixtures of
these resins. Among them, cellulosic, vinylacetal, vinylbutyral, and
polyester resins are preferred from the viewpoints of heat resistance and
transferability of dyes and the like.

[0124]The thermal dye sublimation transfer fluorescent dye layer may
optionally contain other ingredients. For example, a release agent, such
as a silicone oil or polyethylene wax, may be incorporated into the
thermal dye sublimation transfer fluorescent dye layer from the viewpoint
of regulating friction between the fluorescent dye layer and the object
or preventing blocking in a wound state.

[0125]As with the thermo-fusible fluorescent ink used in the first method,
the content ratio of the colorless fluorescent agent to the binder resin
in the thermal dye sublimation transfer fluorescent dye layer used in the
second method may be properly determined according to properties
required. The content of the colorless fluorescent agent is preferably
about 0.1 to 80% by weight, particularly preferably about 1 to 50% by
weight, based on the whole fluorescent dye layer. The content of the
binder resin is preferably about 20 to 99.9% by weight, particularly
preferably about 50 to 99% by weight, based on the whole fluorescent dye
layer. The content of the colorless fluorescent agent in the fluorescent
dye layer is preferably larger than the content of the colorless
fluorescent agent in the thermo-fusible fluorescent ink layer. The reason
for this is as follows. In the case of thermal dye sublimation transfer,
the fluorescent agent in the transfer layer is not completely transferred
onto the object, and a part of the fluorescent agent stays in the
transfer layer. Therefore, in order to form a sharp fluorescent color
image, the content of the fluorescent agent in the transfer layer should
be large.

[0126]In the formation of the thermal dye sublimation transfer fluorescent
ink layer, the ratio between a plurality of colorless fluorescent agents
incorporated is not particularly limited, and, in order to provide a
desired color tone, two, three or more colorless fluorescent agents may
be incorporated at any desired ratio.

[0127]As with the thickness of the thermo-fusible fluorescent ink layer,
the thickness of the fluorescent dye layer is generally 0.2 to 5 μm,
preferably 0.4 to 3 μm.

[0128]The fluorescent dye layer may be formed on the substrate film by
dissolving or dispersing the colorless fluorescent agent, the binder
resin and optionally other ingredients in a single solvent or a mixed
solvent composed of two or more solvents selected from toluene, methyl
ethyl ketone, ethyl acetate, isopropanol and the like to prepare a
coating liquid, coating the coating liquid onto the substrate film by a
conventional method, such as gravure coating, gravure reverse coating, or
roll coating, and drying the coating.

[0129]In the thermal transfer sheet according to the present invention, in
addition to the fluorescent color transfer layer, colorant transfer
layers of yellow, magenta, cyan, black and the like may be provided in a
face serial manner. Sublimable dye-containing dye layers and
thermo-fusible ink layers may be used as the colorant transfer layer.

Second Thermal Transfer Sheet

[0130]FIG. 3A is a diagram illustrating a basic form of the cross section
of the thermal transfer sheet according to the present invention. As
shown in FIG. 3A, the thermal transfer sheet according to the present
invention comprises a substrate film 1 and, provided on one side of the
substrate film 1 in the following order, a release layer 2, an
intermediate layer 3, and a heat-sensitive adhesive layer 4, both the
intermediate layer 3 and the heat-sensitive adhesive layer 4 containing a
fluorescent agent which emits fluorescence upon exposure to ultraviolet
light.

[0131]Substrate Film:

[0132]The substrate film used in the second thermal transfer sheet
according to the present invention may be any conventional substrate film
which has a certain level of heat resistance and strength. For example,
preferably about 0.5 to 50 μm-thick, more preferably about 3 to 10
μm-thick, papers, various converted papers, polyester films,
polystyrene films, polypropylene films, polysulfone films, polycarbonate
films, aramid films, polyvinyl alcohol films, and cellophane may be
mentioned as the substrate film. Particularly preferred are polyester
films. A heat-resistant slip layer (not shown) may be provided on the
backside of the substrate film from the viewpoint of preventing fusing of
a thermal head to the substrate film.

[0133]Release Layer:

[0134]A release layer for facilitating the separation of the intermediate
layer and the heat-sensitive adhesive layer from the substrate film is
provided on the substrate film. Examples of resins usable in the release
layer include: acrylic resins; urethane resins; acrylic resins and
urethane resins which have been modified with silicone; polyvinyl acetal
resins; polyvinyl alcohol resins; and mixtures of the above resins. The
release layer may be formed by dissolving the resin in a solvent to
prepare a coating liquid, coating the coating liquid and drying the
coating. The thickness of the release layer is about 0.1 to 5.0 μm.

[0135]Intermediate Layer:

[0136]The intermediate layer is a layer which is located as the uppermost
layer after transfer. The intermediate layer may be formed of various
resins having excellent fastness properties. Examples of resins usable in
the intermediate layer include: polyester resins; polystyrene resins;
acrylic resins; polyurethane resins; acrylated urethane resins; vinyl
chloride resins; vinyl acetate resins; vinyl chloride/vinyl acetate
copolymer resins; polyamide resins; the above resins modified with
silicone; and mixtures of the above resins. The intermediate layer may be
formed by dissolving or dispersing the resin and the fluorescent agent in
a solvent to prepare a coating liquid, coating the coating liquid, and
drying the coating. The thickness of the intermediate layer is about 0.2
to 5.0 μm. The intermediate layer is preferably colorless and
transparent so that the image covered with the intermediate layer is
visible.

[0137]Heat-Sensitive Adhesive Layer:

[0138]The heat-sensitive adhesive layer is a layer which permits the
intermediate layer to be transferred and adhered to the surface of the
image formed on the image-receiving sheet. The heat-sensitive adhesive
layer is formed of the so-called "heat-sealing resin." Specific examples
thereof include resins having good adhesion upon heating, such as acrylic
resins, vinyl chloride resins, vinyl acetate resins, vinyl chloride/vinyl
acetate copolymer resins, polyester resins, and polyamide resins. The
heat-sensitive adhesive layer may be formed by dissolving or dispersing
the resin and the fluorescent agent in a solvent to prepare a coating
liquid, coating the coating liquid, and drying the coating. The thickness
of the heat-sensitive adhesive layer is about 0.1 to 5 μm. The
heat-sensitive adhesive layer is preferably colorless and transparent so
that the image covered with the heat-sensitive adhesive layer is visible.

[0139]The total thickness of the intermediate layer and the heat-sensitive
adhesive layer is in the range of 0.3 to 10 μm, preferably 0.4 to 5
μm. A total thickness of less than 0.3 μm causes uneven thickness
which is causative of uneven fluorescent color. On the other hand, when
the total thickness exceeds 10 μm, the transferability of the
intermediate layer and the heat-sensitive adhesive layer (hereinafter
often referred to as "fluorescent agent-containing layer") at the time of
transfer is deteriorated. In this case, the transfer of the intermediate
layer and the heat-sensitive adhesive layer onto only a desired region is
difficult, and, in addition, the transfer of the intermediate layer and
the heat-sensitive adhesive layer also onto a region other than the
desired region disadvantageously occurs.

[0140]Fluorescent Agent:

[0141]A large number of conventional organic and inorganic fluorescent
agents are usable as the fluorescent agent in the present invention. In
the present invention, any of conventional fluorescent agents may be
used. However, organic fluorescent agents, which are soluble in the resin
constituting the heat-sensitive adhesive layer and the intermediate layer
and are colorless under normal conditions, are preferred from the
viewpoint of avoiding concealment of the image covered with the
fluorescent agent-containing layer by the fluorescent agent-containing
layer. Organic fluorescent agents usable herein include EB-501, EG-502,
and ER-120 (all of them being tradenames) manufactured by Mitsui
Chemicals Inc., EuN-0001 (tradename) manufactured by Nippon Kayaku Co.,
Ltd., Uvitex OB (tradename) manufactured by Ciba-Geigy, and colorless
fluorescent colorants and various fluorescent brighteners manufactured by
Sinloihi Co., Ltd.

[0142]The amount of the fluorescent agent added to the intermediate layer
and the heat-sensitive adhesive layer may be properly determined
according to properties required and is not particularly limited. When
the fluorescent agent is not highly compatible with the resin for the
intermediate layer and the resin for the adhesive layer, however, a high
fluorescent agent content poses a problem that the fluorescent agent
precipitates in the layer. For this reason, the content of the
fluorescent agent is preferably about 0.01 to 50% by weight, more
preferably about 0.1 to 20% by weight, based on the whole intermediate
layer and heat-sensitive adhesive layer.

[0143]Image-Receiving Sheet:

[0144]The image-receiving sheet, on which an image is formed using the
thermal transfer sheet according to the present invention, may be any
image-receiving sheet such as paper, plastic sheet, or cloth. When a
thermal transfer sheet, wherein sublimable dye layers, which will be
described later, are provided in a face serial manner, is used, the
image-receiving sheet in its image formation face should be dyeable with
a dye. For example, in the case of a paper substrate or the like which is
not dyeable with a dye, a dye-receptive layer formed of a highly dyeable
polyester resin or the like is provided on the non-dyeable substrate.

[0145]Transfer Method:

[0146]In the transfer method using the thermal transfer sheet according to
the present invention, the thermal transfer sheet of the present
invention is put on top of an image-receiving sheet so that the surface
of the heat-sensitive adhesive layer in the thermal transfer sheet faces
the image-receiving sheet, followed by thermal transfer by a conventional
method using a hot press, a heat roll, a thermal printer or the like.
When a fluorescent agent-containing layer is transferred in a pattern
form, a method may be used wherein a fluorescent agent-containing layer
is previously formed in a pattern form. Alternatively, the fluorescent
agent-containing layer may be transferred in a pattern form by providing
a recording device, for example, a thermal printer (for example, a video
printer VY-100, manufactured by Hitachi, Ltd.) and applying a heat energy
regulated at about 5 to 100 mJ/mm2 by controlling the recording
time.

[0147]In the above transfer, an image may be previously formed in the
transfer region, or alternatively any image is not previously formed in
the transfer region. FIG. 3B shows an embodiment wherein a single-color
or full-color image 6 is previously formed on an image-receiving sheet 5
by thermal dye sublimation transfer and the fluorescent agent-containing
layer is transferred so as to cover the image 6. In this image, upon the
application of ultraviolet light to the fluorescent agent contained in
the fluorescent agent-containing layer, the fluorescent agent emits
fluorescence. This significantly changes the hue of the image 6 and thus
makes it difficult to forge or alter the print.

Other Embodiments

[0148]In the present invention, a fluorescent agent is incorporated into
the heat-sensitive adhesive layer and the intermediate layer. The
fluorescent agent incorporated into the heat-sensitive adhesive layer may
be the same as or different from the fluorescent agent incorporated into
the intermediate layer. In this case, a single fluorescent agent may be
used, or a mixture of two or more fluorescent agents may be used. When an
identical fluorescent agent is incorporated into both the heat-sensitive
adhesive layer and the intermediate layer, upon the application of
ultraviolet light, high-intensity fluorescence can be emitted. On the
other hand, when the fluorescent agent incorporated into the
heat-sensitive adhesive layer and the fluorescent agent incorporated into
the intermediate layer are different from each other in fluorescent color
emitted, upon the application of ultraviolet light, fluorescence as a
color mixture of the two fluorescent agents is emitted. In this case, the
forgery/alteration of the print is more difficult.

[0149]In another embodiment of the present invention, as illustrated in
FIG. 3C, a florescent agent-containing layer and, in addition, a single
layer or a plurality of layers selected from sublimable dye layers and
thermo-fusible black ink layers of at least one color selected from
yellow, magenta, cyan, and black colors are formed in a face serial
manner on an identical side of an identical substrate film. In the case
of the thermo-fusible ink layer, the above-described release layer may be
provided between the substrate film and the ink layer.

[0150]In FIG. 3C, all of the colorant layers of yellow (Y), magenta (M),
cyan (C), and black (Bk) may be a sublimable dye layer comprising a
sublimable dye and a binder. Alternatively, all of the colorant layers of
yellow (Y), magenta (M), cyan (C), and black (Bk) may be a thermo-fusible
ink layer comprising a suitable colorant and a wax or a thermoplastic
resin. According to a preferred embodiment of the present invention,
yellow, magenta, and cyan are sublimable dyes, a full-color gradation
image is formed of these three colors, and the black layer is a
thermo-fusible ink layer for the formation of a non-gradation image such
as characters. Sublimable dyes, binders for the sublimable dyes,
thermo-fusible materials such as wax, colorants for the thermo-fusible
materials, and materials for the dye layers and the ink layers and
methods for the formation of these layers usable in this embodiment are
known, and the dye layer and the ink layer may be formed by the
conventional method.

[0151]Further, in the present invention, as shown in FIG. 3C, a
transferable protective layer 7 may be provided in a face serial
relationship with the fluorescent agent-containing layer 8 on an
identical side of the substrate film 1. Regarding material usable for the
protective layer and methods for the formation of the protective layer,
various resins commonly used as resins for protective layers may be used
for the formation of the protective layer. Resins for the protective
layer include, for example, polyester resins, polystyrene resins, acrylic
resins, polyurethane resins, acrylated urethane resins, the above resins
modified with silicone, mixtures of the above resins, and ionizing
radiation-curable resins.

[0152]The transferable protective layer may be formed by dissolving the
resin in a solvent to prepare a coating liquid, coating the coating
liquid, and drying the coating. The thickness of the protective layer is
about 0.5 to 10 μm. FIG. 3D illustrates such a state that fluorescent
agent-containing layers (heat-sensitive adhesive layer 4 and intermediate
layer 3) have been transferred onto the surface of a sublimable dye image
6 followed by the transfer of a protective layer 7 onto the surface of
the intermediate layer 3. The transfer of the protective layer 7 can
improve various fastness properties such as weathering resistance,
chemical resistance, and scratch resistance of the image 6 and the
fluorescent agent-containing layers 3, 4.

Example A

Image Transfer Method According to the Present Invention

[0153]The following examples and comparative examples further illustrate
the present invention. In the following description, "parts" or "%" is by
weight unless otherwise specified.

Preparation of Coating Liquids

[0154]A coating liquid for a heat-resistant layer, a coating liquid for a
release layer, a coating liquid for a fluorescent color transfer layer, a
coating liquid for a thermo-fusible black ink layer, and a coating liquid
for a protective layer were prepared according to the following
formulations.

[0155]The coating liquid for a heat-resistant layer was gravure coated at
a coverage of 0.8 g/m2 on a solid basis onto one side of a 6
μm-thick polyethylene terephthalate film subjected to easy adhesion
treatment, and the coating was dried to form a heat-resistant layer. The
substrate film thus obtained was used to prepare thermal transfer sheets
of respective examples which will be described later.

Example 1A

[0156]The coating liquid for a release layer was gravure coated at a
coverage of 1 g/m2 on a solid basis onto the substrate film, for a
thermal transfer sheet, in its side remote from the heat-resistant layer,
and the coating was dried to form a release layer. Next, the coating
liquid 1 for a thermo-fusible fluorescent color transfer layer (red) was
coated at a coverage of 1 g/m2 onto the release layer, and the
coating was dried to form a fluorescent color transfer layer. Thus, a
thermo-fusible transfer sheet 1 was prepared.

[0157]A thermo-fusible transfer sheet 2 and a thermo-fusible transfer
sheet 3 were prepared in the same manner as described just above, except
that the coating liquid 2 for a thermo-fusible fluorescent color transfer
layer (green) and the coating liquid 3 for a thermo-fusible fluorescent
color transfer layer (blue) were used instead of the coating liquid 1 for
the thermo-fusible fluorescent color transfer layer.

Example 1B

[0158]The coating liquid for a release layer was gravure coated at a
coverage of 1 g/m2 on a solid basis onto the substrate film, for a
thermal transfer sheet, in its side remote from the heat-resistant layer,
and the coating was dried to form a release layer. Next, the coating
liquid 1 for a thermo-fusible fluorescent color transfer layer (red), the
coating liquid 2 for a thermo-fusible fluorescent color transfer layer
(green), and the coating liquid 3 for a thermo-fusible fluorescent color
transfer layer (blue) were gravure coated in that order in a face serial
manner each at a coverage of 1 g/m2 on a solid basis onto the
release layer, and the coatings were dried to form fluorescent color
transfer layers of respective colors. Thus, a thermo-fusible transfer
sheet 4 was prepared. The fluorescent color transfer layers were formed
each in a length of 15 cm along the direction of flow of the substrate
film while leaving a space of 1 cm between adjacent transfer layers.

Example 1C

[0159]A thermo-fusible transfer sheet 5 was prepared in the same manner as
in Example 1B, except that a thermo-fusible black ink layer, together
with the fluorescent color transfer layers of three colors, was formed in
a face serial manner. The thermo-fusible black ink layer was formed by
gravure coating the coating liquid for a thermo-fusible black ink at a
position next to the fluorescent color transfer layers on the release
layer at a coverage of 0.7 g/m2 on a solid basis along the direction
of flow of the substrate film in a length of 15 cm while leaving a space
of 1 cm in the front portion and the rear portion of the thermo-fusible
black ink layer.

Example 1D

[0160]A thermo-fusible transfer sheet 6 was prepared in the same manner as
in Example 1B, except that a transferable protective layer, together with
the fluorescent color transfer layers of three colors, was formed in a
face serial manner. The transferable protective layer was formed by
gravure coating the coating liquid for a protective layer at a position
next to the fluorescent color transfer layers on the release layer at a
coverage of 0.8 g/m2 on a solid basis along the direction of flow of
the substrate film in a length of 15 cm while leaving a space of 1 cm in
the front portion and the rear portion of the transferable protective
layer.

Example 1E

[0161]The coating liquid 4 for a sublimation fluorescent color transfer
layer (red) was gravure coated at a coverage of 0.8 g/m2 on a solid
basis onto the substrate film, for a thermal transfer sheet, in its side
remote from the heat-resistant layer, and the coating was dried to form a
fluorescent color transfer layer. Thus, a thermal dye sublimation
transfer sheet 7 was prepared.

[0162]A thermal dye sublimation transfer sheet 8 and a thermal dye
sublimation transfer sheet 9 were prepared in the same manner as
described just above, except that the coating liquid 5 for a sublimation
fluorescent color transfer layer (green) and the coating liquid 6 for a
sublimation fluorescent color transfer layer (blue) were used instead of
the coating liquid 4 for a sublimation fluorescent color transfer layer.

Comparative Example 1A

[0163]Comparative thermo-fusible transfer sheets 1, 2, and 3 were prepared
in the same manner as in Example 1A, except that the coating liquids 1,
2, and 3 for an inorganic fluorescent color transfer layer were used
instead of the coating liquids 1, 2, and 3 for an organic thermo-fusible
fluorescent color transfer layer.

Evaluation Methods and Results

[0164]The thermal transfer sheets prepared in the above examples and
comparative examples were used to form prints by any one of the following
gradation methods, and the prints were then evaluated. In all the print
tests, L size paper A4 for Color Printer P-400 manufactured by Olympus
Optical Co., LTD. was used as a thermal transfer image-receiving sheet.

(1) Area Gradation Image 1

[0165]A photoretouching software "Photoshop" manufactured by Adobe was
used to prepare a comparative print 2 having an area gradation image 1.
This area gradation image is an area gradation image by a conventional
dither method, and color dots of R, G, and B have portions which have
overlapped with each other or one another.

(2) Area Gradation Image 2

[0166]Next, a print 1A, a print 1B, a print 1C, a print 1D, and a
comparative print 1A each having an area gradation image 2 were prepared
wherein, unlike the above case, color dots of R, G, and B were formed so
as not to overlap with each other.

(3) Density Gradation Image

[0167]In order to carry out the second method according to the present
invention, a print 1E having a density gradation image 1 was prepared by
thermal dye sublimation transfer.

Preparation of Print 1A

[0168]The thermo-fusible transfer sheet 1 prepared in Example 1A was put
on top of the thermal transfer image-receiving sheet. The laminate was
sandwiched between a thermal head and a platen roll, and, while pressing
the laminate between the thermal head and the platen roll, energy was
applied under conditions of 160 mJ/mm2 and printing speed 33.3
msec/line (feed pitch 6 lines/mm). Thereafter, the two sheets were
separated from each other to form an image of a colorless fluorescent
agent on the thermal transfer image-receiving sheet.

[0169]Next, the area gradation image 2 including a mixed portion of the
fluorescent colors was formed in the region, where the image had been
formed using the thermo-fusible transfer sheet 1, in the same manner as
described above, except that the thermo-fusible transfer sheet 2 and the
thermo-fusible transfer sheet 3 were used. The image of colorless
fluorescent agents thus obtained was substantially colorless and was
difficult to visually perceive under visible light. Upon the application
of commercially available black light (emission wavelength 365 nm), the
image formed portion emitted substantially white light and could be
clearly visually perceived. In this case, the color tone obtained was
clearly different from the color tones of red, green, and blue used.

Preparation of Print 1B

[0170]The thermo-fusible transfer sheet 4 prepared in Example 1B was
provided, and fluorescent colors of red, green, and blue were
successively transferred onto the image-receiving sheet in its identical
region under the same printing conditions as used in the preparation of
the print 1A to form the area gradation image 2 including a mixed portion
of the fluorescent colors.

[0171]The image of colorless fluorescent agents thus obtained was
substantially colorless and was difficult to visually perceive under
visible light. Upon the application of commercially available black light
(emission wavelength 365 nm), the color tones of the colorless
fluorescent agents used in the image formation portion were additively
mixed. As a result, full-color light was emitted and could be clearly
visually perceived.

Preparation of Print 1C

[0172]The thermo-fusible transfer sheet 5 prepared in Example 1C was
provided. Black by the thermo-fusible black ink and fluorescent colors of
red, green, and blue were successively transferred onto the
image-receiving sheet in its identical region under the same printing
conditions as used in the preparation of the print 1A to form characters
formed of the thermo-fusible black ink and the area gradation image 1B
including a mixed portion of the fluorescent colors.

[0173]For the print thus obtained, under visible light, only the back
character image derived from the thermo-fusible black ink could be
perceived, and the image appeared to be the same as the conventional
image recorded by thermal transfer. However, upon the application of
commercially available black light (emission wavelength 365 nm), the
color tones of the colorless fluorescent agents were additively mixed in
the fluorescent agent image formed portion. As a result, full color light
was emitted and could be clearly visually perceived.

Preparation of Print 1D

[0174]The thermo-fusible transfer sheet 6 prepared in Example 1D was
provided. Fluorescent colors of red, green, and blue and the transferable
protective layer were successively transferred onto the image-receiving
sheet in its identical region under the same printing conditions as used
in the preparation of the print 1A to form the area gradation image 2
including a mixed portion of the fluorescent colors and, in addition, to
cover the image with a protective layer.

[0175]The image thus obtained was substantially colorless and was
difficult to visually perceive under visible light. Upon the application
of commercially available black light (emission wavelength 365 nm),
however, the color tones of the colorless fluorescent agents were
additively mixed in the image formed portion. As a result, full color
light was emitted and could be clearly visually perceived.

Preparation of Print 1E

[0176]The thermal dye sublimation transfer sheets 7, 8, and 9 prepared in
Example 1E were provided. Fluorescent colors of red, green, and blue were
then successively transferred onto the image-receiving sheet in its
identical region under the same printing conditions as used in the
preparation of the print 1A to form the area gradation image 2 including
a mixed portion of the fluorescent colors.

[0177]The image thus obtained was substantially colorless and was
difficult to visually perceive under visible light. Upon the application
of commercially available black light (emission wavelength 365 nm),
however, a full-color fluorescent image having smooth gradation as
observed in images transferred by the conventional dye sublimation
transfer could be visually perceived in the image formed portion.

Preparation of Comparative Print 1A

[0178]The area gradation image 2 of inorganic colorless fluorescent agents
was formed under the same printing conditions as used in the preparation
of the print 1A, except that the comparative thermo-fusible transfer
sheets 1, 2, and 3 prepared in Comparative Example 1A were used. The
image thus obtained emitted substantially white color under visible
light, and the formation of some image was clearly visually perceived.

[0179]Upon the application of commercially available black light (emission
wavelength 365 nm) to this image, the image formed portion emitted blue
light and could be clearly visually perceived.

Preparation of Comparative Print 1B

[0180]The area gradation image 1 was formed using the comparative
thermo-fusible transfer sheets 1, 2, and 3 under the same conditions as
used in the preparation of the comparative print 1A. The image thus
obtained emitted substantially white color under visible light, and the
formation of some image was clearly visually perceived.

[0181]Upon the application of commercially available black light (emission
wavelength 365 nm) to this image, in the image formed portion, the colors
of R, G, and B and the color tone derived from additive color mixing
could be confirmed. However, no natural full-color image could be
obtained. The fluorescent image was enlarged and observed under a
microscope. As a result, it was found that, in a portion where two colors
or three colors of the transfer layers of R, G, and B were superimposed,
the color development of the lower transfer layer in the superimposed
transfer layers was weak, and, thus, the image was not seen as a natural
image derived from additive color mixing. Further, upon rubbing with a
finger, the portion, where the colors were superimposed, was easily
separated, indicating that the image did not have scratch resistance high
enough to withstand practical use.

[0183]A coating liquid for a heat-resistant layer, a coating liquid for a
release layer, a coating liquid for a fluorescent color transfer layer, a
coating liquid for a thermo-fusible black ink layer, and a coating liquid
for a protective layer were prepared according to the following
formulations. All the coating liquids except for the coating liquids for
fluorescent color transfer layers were the same as those in Example A.

[0184]A substrate film for a thermal transfer sheet was prepared in the
same manner as in Example 1A.

Example 2A

[0185]The coating liquid for a release layer was gravure coated at a
coverage of 1 g/m2 on a solid basis onto the substrate film, for a
thermal transfer sheet, in its side remote from the heat-resistant layer,
and the coating was dried to form a release layer. Next, the coating
liquid 1 for a fluorescent color transfer layer was coated at a coverage
of 1 g/m2 on a solid basis onto the release layer, and the coating
was dried to form a fluorescent color transfer layer. Thus, a thermal
transfer sheet 2A was prepared.

Example 2B

[0186]A thermal transfer sheet 2B was prepared in the same manner as in
Example 2A, except that the coating liquid 2 for a fluorescent color
transfer layer was used instead of the coating liquid 1 for a fluorescent
color transfer layer.

Example 2C

[0187]The coating liquid 1 for a dye layer (yellow), the coating liquid 2
for a dye layer (magenta), the coating liquid 3 for a dye layer (cyan),
and the coating liquid 1 for a fluorescent color transfer layer were
gravure coated in that order in a face serial manner onto the substrate
film, for a thermal transfer sheet, in its side remote from the
heat-resistant layer, each at a coverage of 1 g/m2 on a solid basis,
and the coatings were dried to form dye layers of the individual colors
and the fluorescent color transfer layer. Thus, a thermal transfer sheet
2C was prepared. The dye layers and the fluorescent color transfer layer
were formed each in a length of 15 cm along the direction of flow of the
substrate film while leaving a space of 1 cm between adjacent layers.

Example 2D

[0188]A thermal transfer sheet 2D was prepared in the same manner as in
Example 2C, except that a thermo-fusible black ink layer, together with
the dye layers and the fluorescent color transfer layer, was formed in a
face serial manner. The coating liquid for a release layer was gravure
coated onto a portion located between the dye layer (cyan) and the
fluorescent color transfer layer on the surface of the substrate film at
a coverage of 1 g/m2 on a solid basis, and the coating was dried to
form a release layer. Thereafter, the coating liquid for a thermo-fusible
black ink layer was gravure coated onto the release layer at a coverage
of 0.7 g/m2 on a solid basis, and the coating was dried to form a
thermo-fusible black ink layer. As with the other transfer layers, the
multilayer structure portion composed of the release layer and the
thermo-fusible black ink layer was formed along the direction of flow of
the substrate film in a length of 15 cm while leaving a space of 1 cm in
the front portion and the rear portion of the multilayer structure
portion.

Example 2E

[0189]A thermal transfer sheet 2E was prepared in the same manner as in
Example 2C, except that a transferable protective layer, together with
the dye layers and the fluorescent color transfer layer, was formed in a
face serial manner. The coating liquid for a release layer was gravure
coated onto a portion located next to the fluorescent color transfer
layer on the surface of the substrate film at a coverage of 1 g/m2
on a solid basis, and the coating was dried to form a release layer.
Thereafter, the coating liquid for a protective layer was gravure coated
onto the release layer at a coverage of 0.8 g/m2 on a solid basis,
and the coating was dried to form a transferable protective layer. As
with the other transfer layers, the multilayer structure portion composed
of the release layer and the transferable protective layer was formed
along the direction of flow of the substrate film in a length of 15 cm
while leaving a space of 1 cm in the front portion and the rear portion
of the multilayer structure portion.

Comparative Example 2A

[0190]A comparative thermal transfer sheet 2F was prepared in the same
manner as in Example 2A, except that the coating liquid 3 for a
fluorescent color transfer layer (single color of blue) was used instead
of the coating liquid 1 for a fluorescent color transfer layer.

Comparative Example 2B

[0191]A comparative thermal transfer sheet 2G was prepared in the same
manner as in Example 2A, except that the coating liquid 4 for a
fluorescent color transfer layer (single color of red) was used instead
of the coating liquid 1 for a fluorescent color transfer layer.

Comparative Example 2C

[0192]A comparative thermal transfer sheet 2H was prepared in the same
manner as in Example 2A, except that the coating liquid 5 for a
fluorescent color transfer layer (single color of green) was used instead
of the coating liquid 1 for a fluorescent color transfer layer.

Evaluation Methods and Results

[0193]The thermal transfer sheets prepared in the above examples and
comparative examples were used to form prints under the following
conditions, and the prints were then evaluated. In all the print tests, L
size paper A4 for Color Printer P-400 manufactured by Olympus Optical
Co., LTD. was used as a thermal transfer image-receiving sheet.

Preparation of Print 2A

[0194]The thermal transfer sheet 2A prepared in Example 2A was put on top
of the thermal transfer image-receiving sheet. The laminate was
sandwiched between a thermal head and a platen roll, and, while pressing
the laminate between the thermal head and the platen roll, energy was
applied under conditions of 160 mJ/mm2 and printing speed 33.3
msec/line (feed pitch 6 lines/mm). Thereafter, the two sheets were
separated from each other to form an image of a colorless fluorescent
agent on the thermal transfer image-receiving sheet.

[0195]The image of colorless fluorescent agents thus obtained was
substantially colorless and was difficult to visually perceive under
visible light. Upon the application of commercially available black light
(emission wavelength 365 nm), however, the image formed portion emitted
substantially white light and could be clearly visually perceived.

Preparation of Print 2B

[0196]The thermal transfer sheet 2B prepared in Example 2B was provided,
and an image of the colorless fluorescent agents was formed on the
thermal transfer image-receiving sheet under the same printing conditions
as used in the preparation of the print 2A.

[0197]The image of colorless fluorescent agents thus obtained was
substantially white under visible light, and the presence of the printed
image could be perceived at some viewing angle. In this case, however, it
was difficult to perceive the detailed fine pattern and the like. Upon
the application of commercially available black light (emission
wavelength 365 nm) to this fluorescent color image, the image formed
portion emitted substantially white light and could be clearly visually
perceived.

Preparation of Print 2C

[0198]The thermal transfer sheet 2C prepared in Example 2C was provided,
and sublimable dyes of yellow, magenta, and cyan and a fluorescent color
as a mixed color were successively transferred onto the image-receiving
sheet in its identical region to form a visible image and a fluorescent
color image.

[0199]The thermal transfer sheet 2C was put on top of the thermal transfer
image-receiving sheet. The laminate was sandwiched between a thermal head
and a platen roll, and, while pressing the laminate between the thermal
head and the platen roll, printing of Y, M, and C was carried out by
applying energy from the backside of the thermal transfer sheet 3 under
conditions of head applied voltage 12.0 V, pulse width 16 msec, printing
period 33.3 msec, and dot density 6 dots/line to form a full-color image.
Thereafter, an image of colorless fluorescent agents was formed in the
identical image formation region under the same printing conditions as
used in the preparation of the print 1.

[0200]For the image thus obtained, only the full-color image derived from
the sublimable dyes could be perceived under visible light, and the image
appeared to be the same as the conventional image recorded by thermal
transfer. Upon the application of commercially available black light
(emission wavelength 365 nm) to this image, however, the image formed
portion emitted substantially white light and could be clearly visually
perceived.

Preparation of Print 2D

[0201]The thermal transfer sheet 2D prepared in Example 2D was provided,
and sublimable dyes of yellow, magenta, and cyan, a thermo-fusible black
ink, and a fluorescent color as a mixed color were successively
transferred onto the image-receiving sheet in its identical region to
form a full-color visible image, a black character image, and a
fluorescent color image.

[0202]The thermal transfer sheet 2D was put on top of the thermal transfer
image-receiving sheet. Printing of Y, M, and C was carried out under the
same printing conditions as used in the preparation of the print 2C to
form a full-color image. Thereafter, the thermo-fusible black ink was
printed in the identical image formation region under conditions of 120
mJ/mm2 and printing speed 33.3 msec/line (feed pitch 6 lines/mm) to
form a black character image. An image of colorless fluorescent agents
was then formed in the identical image formation region under the same
printing conditions as used in the preparation of the print 1.

[0203]For the image thus obtained, only the full-color image derived from
the sublimable dyes and the black character image derived from the
thermo-fusible black ink could be perceived under visible light, and the
image appeared to be the same as the conventional image recorded by
thermal transfer. Upon the application of commercially available black
light (emission wavelength 365 nm) to this image, however, the image
formed portion emitted substantially white light and could be clearly
visually perceived.

Preparation of Print 2E

[0204]The thermal transfer sheet 2E prepared in Example 2E was provided,
and sublimable dyes of yellow, magenta, and cyan and a fluorescent color
as a mixed color were successively transferred onto the image-receiving
sheet in its identical region to form a full-color visible image and a
fluorescent color image, and, in addition, a protective layer was
transferred onto the formed image to cover the image with the protective
layer.

[0205]The thermal transfer sheet 2E was put on top of the thermal transfer
image-receiving sheet. Printing of Y, M, and C was carried out under the
same printing conditions as used in the preparation of the print 2C to
form a full-color image. Thereafter, an image of colorless fluorescent
agents was formed in the identical image formation region under the same
printing conditions as used in the preparation of the print 1. A
transferable protective layer was then thermally transferred under
conditions of 160 mJ/mm2, printing speed 33.3 msec/line (feed pitch
6 lines/mm) to cover the image with the protective layer.

[0206]For the image thus obtained, only the full-color image derived from
the sublimable dyes could be perceived under visible light, and the image
appeared to be the same as the conventional image recorded by thermal
transfer. Upon the application of commercially available black light
(emission wavelength 365 nm) to this image, however, the image formed
portion emitted substantially white light and could be clearly visually
perceived.

Preparation of Comparative Print 2F

[0207]An image of a colorless fluorescent agent was formed under the same
printing conditions as used in the preparation of the print 2A, except
that the thermal transfer sheet 2F prepared in Comparative Example 2A was
used.

[0208]The image of the colorless fluorescent agent thus obtained was
substantially colorless and was difficult to visually perceive under
visible light, and, upon the application of commercially available black
light (emission wavelength 365 nm), the image formed portion emitted blue
light and could be clearly visually perceived. The color tone of the
fluorescent color emitted from the image, however, was the color tone per
se of the fluorescent agent incorporated into the fluorescent color
transfer layer.

Preparation of Comparative Print 2G

[0209]An image of a colorless fluorescent agent was formed under the same
printing conditions as used in the preparation of the print 2A, except
that the thermal transfer sheet 2G prepared in Comparative Example 2B was
used.

[0210]The image of the colorless fluorescent agent thus obtained was
substantially colorless and was difficult to visually perceive under
visible light, and, upon the application of commercially available black
light (emission wavelength 365 nm), the image formed portion emitted red
light and could be clearly visually perceived. The color tone of the
fluorescent color emitted from the image, however, was the color tone per
se of the fluorescent agent incorporated into the fluorescent color
transfer layer.

Preparation of Comparative Print 2H

[0211]An image of a colorless fluorescent agent was formed under the same
printing conditions as used in the preparation of the print 2A, except
that the thermal transfer sheet 2H prepared in Comparative Example 2C was
used.

[0212]The image of the colorless fluorescent agent thus obtained was
substantially colorless and was difficult to visually perceive under
visible light, and, upon the application of commercially available black
light (emission wavelength 365 nm), the image formed portion emitted
green light and could be clearly visually perceived. The color tone of
the fluorescent color emitted from the image, however, was the color tone
per se of the fluorescent agent incorporated into the fluorescent color
transfer layer.

Example C

Thermal Transfer Sheet According to Second Invention

Preparation of Substrate Film 1

[0213]The following coating liquid for a heat-resistant slip layer was
gravure coated on the surface of a 6 μm-thick polyester film at a
coverage of 0.5 μm, and the coating was dried. Thus, a substrate film
1 was prepared.

[0214]A coating liquid for a release layer, a coating liquid for an
intermediate layer, and a coating liquid for a heat-sensitive adhesive
layer were prepared according to the following formulations. The coating
liquid for a release layer, the coating liquid for an intermediate layer,
and the coating liquid for a heat-sensitive adhesive layer were
successively gravure coated onto the substrate film 1 in its side remote
from the heat-resistant slip layer respectively at coverages of 0.5
μm, 1.0 μm, and 1.0 μm, and the coatings were dried and stacked
to form a thermal transfer sheet of the present invention.

[0215]A thermal transfer sheet according to the present invention was
prepared in the same manner as in Example 3A, except that only the
composition of the coating liquid for a heat-sensitive adhesive layer was
changed to the following composition.

[0216]A coating liquid for a yellow ink layer, a coating liquid for a
magenta ink layer, a coating liquid for a cyan ink layer, and a coating
liquid for a black ink layer were prepared according to the following
formulations, and the coating liquid for a yellow ink layer, the coating
liquid for a magenta ink layer, the coating liquid for a cyan ink layer,
and the coating liquid for a black ink layer were coated in a face serial
manner each at a coverage of 1.0 μm on the substrate film 1 in this
side remote from the heat-resistant slip layer, and the coatings were
dried to form a substrate film 2.

[0217]A thermal transfer sheet of the present invention was prepared in
the same manner as in Example 3A, except that the substrate film 1 was
changed to the substrate film 2 and, after the formation of the black ink
layer, the release layer, the intermediate layer, and the heat-sensitive
adhesive layer were successively stacked by coating and drying.

Example 3D

[0218]A thermal transfer sheet of the present invention was prepared in
the same manner as in Example 3B, except that the substrate film 1 was
changed to the substrate film 2 and, after the formation of the black ink
layer, the release layer, the intermediate layer, and the heat-sensitive
adhesive layer were successively stacked by coating and drying.

Example 3E

[0219]A thermal transfer sheet of the present invention was prepared in
the same manner as in Example 3D, except that, after the formation of the
fluorescent agent-containing layer, a coating liquid for a release layer
having the following composition and a coating liquid for a protective
layer having the following composition were coated respectively at
coverages of 0.5 μm and 1.0 μm, and the coatings were dried.

[0220]A thermal transfer sheet of the present invention was prepared in
the same manner as in Example 3D, except that, after the formation of the
black ink layer, a fluorescent agent-containing layer was coated onto the
center portion of the film to a coating area of one-eighth of the coating
area of the black ink layer.

Comparative Example 3A

[0221]A transfer sheet of Comparative Example 3A was prepared in the same
manner as in Example 3A, except that only the composition of the coating
liquid for a heat-sensitive adhesive layer was changed to the following
composition.

[0222]Evaluation was carried out using a printer P-330 manufactured by
Olympus Optical Co., LTD. A thermal transfer image-receiving sheet
included as a set in P-330 was used as the printing paper.

[0223]1) The thermal transfer sheet prepared in Example 3A was put on top
of the thermal transfer image-receiving sheet to adhere the fluorescent
agent-containing layer to the thermal transfer image-receiving sheet, and
the base film was then separated to form an image of the colorless
fluorescent agent-containing layer on the thermal transfer
image-receiving sheet. The image of the colorless fluorescent
agent-containing layer was substantially colorless and was difficult to
visually perceive under visible light. Upon the application of
commercially available black light (emission wavelength 365 nm), the
image formed portion emitted blue light and could be clearly visually
perceived.

[0224]2) The thermal transfer sheet prepared in Example 3B was put on top
of the thermal transfer image-receiving sheet to adhere the fluorescent
agent-containing layer to the thermal transfer image-receiving sheet, and
the base film was then separated to form an image of the colorless
fluorescent agent-containing layer on the thermal transfer
image-receiving sheet. The image of the colorless fluorescent
agent-containing layer was substantially colorless and was difficult to
visually perceive under visible light. Upon the application of
commercially available black light (emission wavelength 365 nm), the
image formed portion emitted a color light of a color mixture of blue and
green and could be clearly visually perceived.

[0225]3) The thermal transfer sheet prepared in Example 3C was put on top
of the thermal transfer image-receiving sheet to adhere the fluorescent
agent-containing layer to the thermal transfer image-receiving sheet.
Thereafter, a full-color natural picture was printed using yellow,
magenta, cyan, and black colors, and an image of a colorless fluorescent
agent-containing layer was formed thereon. For the image-received sheet
thus obtained, under visible light, only the natural picture could be
visually perceived, and the image of the colorless fluorescent
agent-containing layer was substantially colorless and was difficult to
visually perceive. Upon the application of commercially available black
light (emission wavelength 365 nm), the image portion composed of the
colorless fluorescent agent-containing layer emitted blue light and could
be clearly visually perceived.

[0226]4) The thermal transfer sheet prepared in Example 3D was put on top
of the thermal transfer image-receiving sheet to adhere the fluorescent
agent-containing layer to the thermal transfer image-receiving sheet.
Thereafter, a full-color natural picture was printed using yellow,
magenta, cyan, and black colors, and an image of a colorless fluorescent
agent-containing layer was formed thereon. For the image-received sheet
thus obtained, under visible light, only the natural picture could be
visually perceived, and the image of the colorless fluorescent
agent-containing layer was substantially colorless and was difficult to
visually perceive. Upon the application of commercially available black
light (emission wavelength 365 nm), the image formed portion emitted a
color light of a color mixture of blue and green and could be clearly
visually perceived.

[0227]5) The thermal transfer sheet prepared in Example 3E was put on top
of the thermal transfer image-receiving sheet to adhere the fluorescent
agent-containing layer to the thermal transfer image-receiving sheet.
Thereafter, a full-color natural picture was printed using yellow,
magenta, cyan, and black colors, and an image of a colorless fluorescent
agent-containing layer was formed thereon. Further, a protective layer
was transferred thereon to cover the whole image. Under visible light,
only the natural picture could be visually perceived, and the image of
the colorless fluorescent agent-containing layer was substantially
colorless and was difficult to visually perceive. Upon the application of
commercially available black light (emission wavelength 365 nm), the
image formed portion emitted a color light of a color mixture of blue and
green and could be clearly visually perceived. Even rubbing of the print
with an eraser several times caused neither discoloration of the image
nor disappearance of the color of the image.

[0228]6) The thermal transfer sheet prepared in Example 3F was put on top
of the thermal transfer image-receiving sheet to adhere the fluorescent
agent-containing layer to the thermal transfer image-receiving sheet.
Thereafter, a full-color natural picture was printed using yellow,
magenta, cyan, and black colors, and an image of a colorless fluorescent
agent-containing layer was formed on the center portion of the sheet. For
the image-received sheet thus obtained, under visible light, only the
natural picture could be visually perceived, and the image of the
colorless fluorescent agent-containing layer was substantially colorless
and was difficult to visually perceive. Upon the application of
commercially available black light (emission wavelength 365 nm), the
image formed portion at the center portion of the sheet emitted a color
light of a color mixture of blue and green and could be clearly visually
perceived.

[0229]7) The thermal transfer sheet prepared in Comparative Example 3A was
put on top of the thermal transfer image-receiving sheet to adhere the
fluorescent agent-containing layer to the thermal transfer
image-receiving sheet, and the base film was then separated to form an
image of the colorless fluorescent agent-containing layer on the thermal
transfer image-receiving sheet. The image of the colorless fluorescent
agent-containing layer was substantially colorless and was difficult to
visually perceive under visible light. Upon the application of
commercially available black light (emission wavelength 365 nm), the
image formed portion emitted blue light. However, the emitted light
intensity was low, and the visibility of the image was poor.